It is believed the extreme recent weather contributed to the erosion, with a period of significant heavy rainfall followed by warm temperatures and sun today.
The woman is understood to have been walking underneath the cliff at the time of the landslip.
All coastal paths and beaches between Lyme Regis and West Bexington have now been closed.
Witness Gary Rafferty, 36, from Bournemouth, said both the woman's boyfriend and father were partially trapped under the mass of rocks but were able to free themselves before attempting a rescue.
Mr Rafferty said: "I was on the top of the cliff at about 12.30pm when I heard a rumble. I rushed down on to the beach and saw this enormous landslide on the beach.
"I rushed to help and helped a man aged in his 50s out of the debris.
"I saw his son who had also been trapped. I said to him 'are you all right' and he said 'no, my girlfriend's trapped under there.' He was quite hysterical.
"Then the lifeboat came and took him and his father off the beach. It must have been very difficult to leave the woman trapped under the rocks.
"They said they had just been walking along the beach and they didn't hear anything. Then all of a sudden there was this huge landslide which fell down on them.
"It was the first landslide that trapped the woman, the father and the boyfriend were on top trying to get her out, I could see them trying to move rocks. They were clearly in shock and were frantically pulled the rocks out of the way.
"Then there was the second landslide and luckily they got out of the way, but the father has done his shoulder in you could see on his back where a falling rock hit it.
"They were very unlucky and were in the wrong place at the wrong time. There were lots of people trying to help but there was nothing anyone could do because the pile of rocks and soil was just so huge.
"They told me they were just walking down the beach when the first landslide came down. I think they were holidaymakers and they both looked shocked."
Two young boys, believed to be aged about 12 and 14, were seen wrapped in silver foil blankets being led from the beach. It is thought they were with their mother who was not involved in the landslide.
Members of the public, including families visiting the beach, raised the alarm at around 12.30 this afternoon.
More than 30 firefighters arrived at the scene but the instability of both the cliff face and the huge pile of mud meant that it was too dangerous to try and reach the woman.
A spokesman for the Dorset Fire and Rescue Service said they were consulting geologists about the stability of the cliffs before they begin to search under the rocks. Rescue teams were willing to work into the night under lights, he added.
Sniffer dogs, heat sensors and listening devices have all been utilised to find the woman under the 35ft high and 70ft wide mound.
One witness described hearing the sound of "thunder" as two landslides occurred within approximately 20 minutes earlier today.
The resident, named as Len Muggeridge, told Sky News there had been two separate landslides, with several members of the public "clambering" over the rocks before the second 100 ton fall.
He said: "The first thing we heard was like thunder. The next thing we could see dust and debris.
"Before the second fall, people had gathered to see what the first fall was like. I think there were people clambering over the rocks when it happened."
Mr Muggeridge, who was fishing at the time of the rock fall, added: "It is the weather. It is basically sandstone all along there and unfortunately it is quite soft.
"Over the last few months we have had a lot of wet weather and it will have got extremely wet. Now it has started to dry out and just come away."
Another witness said he feared there was "no hope" for anybody trapped underneath all the rocks.
The area is popular with tourists, with many fossil hunters visiting the Jurassic cliffs and beach in search of dinosaur remains.
At least 150 holidaymakers were estimated to have been on the beach enjoying the hot sunshine at the time of the landslides.
Dave Smith, 42, said: "I was fishing on the beach and all of a sudden I heard a great rumble and there was a huge amount of dust. I saw part of the cliff had slumped into the sea.
"I didn't [see if] anyone was under there at the time but people do tend to sit under the cliff but I expect all the rain we have had followed by the hot spell has caused the cliff to crumble."
Mick Stead, a group manager with the Dorset Fire and Rescue Service, said: "There was a significant landslip, about a 20 metre stretch of the coastal path has given way.
"We estimate that about 400 tons of mud and rock has fallen from the top and down onto the beach. The pile of mud is about 10 metres at its highest point and 20 metres wide.
"We are working on the presumption that there is one person trapped underneath it.
"We have got sniffer dogs from Devon and Somserset Fire and Rescue searching the pile of mud but we haven't ben able to get close to it ourselves.
"We have assessed the risks to the emergency workers and it is a question of trying to balance the rescue operation against the risk to us."
He added the recent heavy rain followed by warm temperatures today could have affected the geology of the area, adding it was an "avenue" they would be looking at when attempting to establish the cause of the landslide.
Graham Wiffen, who works at a nearby beach café, said: "There are lots of signs up there telling people not to stand underneath the cliffs but everybody does it.
"As you can image the beach is very busy what with it being the height of the summer. It is the nature of the Jurassic coast that it erodes and these landslips are unfortunately common."
One female member of staff at the Anchor Public House, Burton Bradstock said: "A couple of hours ago the first helicopter went up and now there are three. Now all we can hear is sirens constantly screaming past.
"A local fisherman has told us that there was a landslip and that it looks quite bad."
Just four days ago, Dorset Council posted a statement on their website warning walkers and fossil hunters to "beware of landslides and other hazards" following recent bad weather.
It states the "exceptionally wet weather of recent months" had led to a "heightened risk of rock falls anywhere and at any time along the coast."
A spokesman added on July 20th: "The advice is to stay well away from the cliffs at all times and to beware of mudflows and quicksand, especially when the tide is coming in as it is possible to become cut off from the normal exit points to and from the beaches."
A spokeswoman for South Western Ambulance Service said this afternoon: "The Trust dispatched a number of vehicles to the Burton Bradstock area in Bridport, Dorset following a number of calls made to the service just after 12.30 today, including the Dorset and Somerset Air Ambulance and the service's Hazardous Area Response Team (HART), who are trained to deal with a whole manner of challenging scenes.
"One person is reported as being trapped and the ambulance service is working in partnership with the other agencies on the scene to reach them as quickly as possible."
A joint statement from Dorset Police and Dorset Fire and Rescue Service issued this afternoon reads: "At just after 12.30pm today, Dorset Police were called to a report from the coastguard of a landslide by Freshwater Beach holiday park, near Burton Bradstock, Bridport.
"Officers, with assistance from the police helicopter, are currently working alongside Dorset Fire and Rescue Service to search the area.
"The Urban Search and Rescue team from Devon and Somerset have also been deployed to this incident."
The official website for the Jurassic Coast explains that the coast is unstable because of rain water sinking through porous rocks.
It says: "Once it reaches the underlying clays it can sink no further. The water builds up along the junction between the rock layers and seeps out of the cliffs as a series of springs.
"After periods of prolonged rainfall, the build up of water increases the weight of the cliff top. Increased pore pressure reduces the friction and allows large sections of the cliff top to break away. As the cliff top block subsides, it rotates along the slip plane within the cliff, resulting in the flat surface tipping back towards the cliff.
"The displacement shunts thousands of tonnes of material into the undercliffs, the area between the cliff top and the beach, generating mudslides within the softer Jurassic clays that in turn slide towards and across the beach."
The area is part of the 95-mile Jurassic Coast along Dorset and east Devon with rocks recording 185 million years of the Earth's history.
It was England's first natural Unesco (United Nations Educational, Scientific and Cultural Organisation) World Heritage Site and clearly depicts a geological "walk through time" spanning the Triassic, Jurassic and Cretaceous periods.
A spokesman for the National Trust, which owns the stretch of beach, said: "Our thoughts are with all those involved in this tragic accident.
"Coastlines are dynamic and changing environments and it is impossible to predict when these kinds of events might occur.
"National Trust land at Burton Bradstock will remain closed whilst we assist the emergency services in whatever way we can."
Last week, an elderly couple died after their car was buried under a landslide in Beaminster, Dorset.
The bodies of Rosemary Snell, 67, and Michael Rolfe, 72, were trapped for ten days before their vehicle was found under a pile of earth by the partially-collapsed Beaminster Tunnel.
In February this year, a couple narrowly missed being crushed to death by 300 tonnes of rock came crashing off a cliff at the same beach.
Tim Cambridge 45, and his pregnant partner Tamara Pollard were just a few feet away when the rocks fell while they were on holiday. Trainee teacher Tamara had paused to pick up a shell on the beach.
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