Hundreds of thousands of protesters filled the streets in cities across Syria on Friday, mounting some of the country's largest anti-government demonstrations yet under the gaze of observers from the Arab League.

Both encouraged and rankled by the observers who arrived on Tuesday, defiant protesters called for the downfall of President Bashar al-Assad, leading to clashes with security forces that left at least 22 dead, according to reports.

The crowds are said to have been the largest in Idlib and Hama provinces at about 250,000 people each. Large rallies also took over parts of Daraa province and Douma, a suburb of Damascus.

"Whether we like it or not, the presence of observers has had a positive psychological effect, encouraging people to stage peaceful protests -- a basic condition of the Arab League peace plan," said Haytham Manna, a Paris-based dissident and human rights advocate.

The size of the protests cannot be verified because Syria has banned most foreign journalists and maintains a tight grip on local media.

Meanwhile, the rebel army said Friday it has suspended its attacks against government forces for the duration of the observers' month-long visit.

The goal, said leader Col. Riad al-Asaad, is to expose the regime's tactics of murdering peaceful demonstrators.

"We stopped to show respect to Arab brothers, to prove that there are no armed gangs in Syria, and for the monitors to be able to go wherever they want," al-Asaad, a defected air force colonel, said from his base in Turkey. "We only defend ourselves now. This is our right and the right of every human being."

About 15,000 members of the Syrian military have abandoned their posts to fight with the Free Syrian Army (FSA). The organization says it's responsible for attacks that have killed Syrian soldiers and security officials, and damaged government property.

The United Nations estimates that more than 5,000 people have been killed in the chaos in Syria since March. The government claims the nine-month crisis is the fault of terrorism and gang violence.

Opposition forces have mixed feelings about the Arab League's observers, whose arrival represents a chance to be heard across the region. But the observer deal has also been criticized as a ploy by Damascus to buy time as governments around the world continue to condemn the country's treatment of its citizens.

Additionally, many of the countries that sent observers are known for human rights violations themselves, leading to suspicion they may not be the most qualified to monitor the regime's compliance with its promise to end the crackdown.

Mousab Azzawi, chairman of the Tunisia-based Arab Institute for Human Rights, said Friday his group finds it "very worrying" that the observers are being led by Lt. Gen. Mohamed Ahmed Mustafa al-Dabi, a former military intelligence chief with close ties to Sudanese president Omar al-Bashir.

Al-Dabi is wanted for questioning by the International Criminal Court in connection with mass killings in Sudan's Darfur region.

"We have very big concerns about the character of this person," Azzawi told CTV News Channel. "We are very skeptical why this person was elected to be the chief of the mission."

The orange-jacketed observers have been seen taking pictures of the destruction, visiting families of victims of the crackdown, and taking notes.

On Friday, they were within "hearing distance" when troops opened fire on protesters in Douma, said activist Salim al-Omar. They later visited the wounded in hospital, he added.

Amateur video of the clashes in Douma, posted online by activists, showed demonstrators carrying away a bleeding comrade after being hit by a gas canister.

"Look, Arab League, look!" the cameraman is heard shouting.

With files from Associated Press