Kept at bay by gunfire, the observers were not able to see areas of Homs that had suffered from the worst violence, and also appeared limited by government efforts to conceal its crackdown. Human Rights Watch reported on Tuesday that the Syrian government had transferred hundreds of prisoners in Homs to other sites, including military installations, where the observers were not allowed to visit. A group of observers was also prevented from visiting the restive town of Dara'a in the south, because of security restrictions, according to a team member.

"The observers are frustrated with the tight monitoring," said a team member who asked not to be named because he is not authorized to speak with the news media.

About 60 observers from Arab League countries arrived in Syria on Monday in the midst of some of the worst violence in months. The group has quickly become the center of critical attention. The government and its opponents have called on the observers to arbitrate an increasingly bloody conflict that journalists and human rights activists have been barred from witnessing.

Some opposition groups have been quick to attack the mission as fatally flawed and a way for the Syrian government to stall for time before it continues its crackdown. Protesters, though, have argued for patience as the observers start their work.

Human rights activists in the region are questioning whether the observers have the freedom to move around Syria, or the qualifications to investigate human rights abuses. The Arab League has yet to release the list of members, but activists say many are diplomats or Arab League employees with little or no experience interviewing victims or documenting war crimes.

Those concerns gained urgency with a video that activists posted on Tuesday, in which residents of the Baba Amr neighborhood — one of the hardest hit by Syrian security forces — begged a group of observers at the entrance of the neighborhood to go further. Tanks could be seen in an empty lot, and a few gunshots could be heard.

"Go inside," one man said. "Go see what's inside. They're massacring us."

They did not go. Later, the leader of the Arab League mission, Lt. Gen. Muhammed al-Dabi of Sudan, told Reuters: "Today was very good and all sides were responsive."

Others disagreed.

"This is the most messed-up mission I have ever seen in my life," said Ziad Abdel Tawab, the deputy director of the Cairo Institute for Human Rights Studies, a group that was asked to participate but has not yet been able to. "I've worked with the United Nations before. There is no mission of this size that can be dispatched in 72 hours. The Arab League is not an institution that even has a history of fact-finding missions."

The league said that it initially planned to have 500 observers go to Syrian cities to monitor the terms of an initiative that asked the government to remove its weapons and soldiers from residential areas, release prisoners and eventually start a dialogue with opposition groups. But a few weeks ago, the secretary general of the Arab League, Nabil al-Araby, said the organization only had 100 observers at its disposal.

On Monday, only 60 observers traveled to the country. Opposition figures say that is nowhere near enough to ensure President Bashar al-Assad's compliance.

Alaa Shalaby, the executive director of the Arab Organization for Human Rights, who is participating in the mission, said that while it was true that the observers had less preparation than they required — a few hours, as opposed to a month — the team included at least 24 experienced human rights workers. He said that other observers would soon join the team. He said the trip to Homs was a first visit and that the monitors would spend more time there, establishing a center under the Arab League flag.

Residents said that some tanks had withdrawn from the streets on Tuesday, though in many places they were simply replaced by soldiers at checkpoints. Gunfire continued as the observers met with the governor of Homs, and then with activists. In video shown by Al Jazeera, tens of thousands of people could be seen demonstrating in a square.

Violence was reported elsewhere in the country on Tuesday. The Syrian state news agency, SANA, reported that "terrorists" had attacked a gas pipeline in Homs Province. They also reported that the security forces had killed several people in northwestern Syria, near the Turkish border, where armed defectors based in Turkey have been trying to create a stronghold. And they said that six workers traveling on the Aleppo-Lattakia road were killed after two roadside bombs detonated under their bus.

Opposition groups said more than 30 people were killed across the country.

On Tuesday, Human Rights Watch said that evidence was emerging that military personnel were being given police identification cards, possibly allowing the government to say it had withdrawn the military from cities.

Despite those challenges, some of the government's opponents said the observer mission should be given a chance. Haytham al-Manna, a Syrian dissident and human rights activist whose brother was killed in August by the security forces, said he met with observers before they traveled to Damascus and suggested that some of the criticism was too hasty. He said that some in the Syrian opposition were determined to "bury the Arab initiative before its even applied."

"How can they punish these observers before the mission has lasted 24 hours?" he said.

Huwaida Saad and an employee of The New York Times contributed reporting from Beirut, and David D. Kirkpatrick from Cairo.