Tens of thousands of protesters rallied near Damascus on Thursday as Syrian security forces killed at least 25 civilians nationwide and peace monitors spread out to areas hit by unrelenting violence.
Some 30,000 people gathered in a square outside the Grand Mosque in Douma just north of the capital, prompting security forces to pull back from previously held positions, the Syrian Observatory for Human Rights said.
Earlier, as Arab League observers arrived at Douma city hall, security forces opened fire on protesters outside the mosque, killing at least four and wounding several others, the rights group said.
The observers, on the third day of a mission aimed at halting the bloodshed in Syria, also visited the central city of Hama, Idlib in the northwest, and Daraa in the south, according to Syrian television.
SANA, the official news agency, said observers visited the Baba Amro neighbourhood of Homs, Harasta city, Daraa and Hama provinces "and met a number of citizens."
Daraa is the cradle of the unprecedented nine-month protest movement against President Bashar al-Assad's regime, which has ruled Syria with an iron fist for 11 years.
According to UN estimates announced in early December, more than 5,000 people have been killed in the Syrian government crackdown on dissent since mid-March.
Activists say that more than 70 civilians have been killed by security forces since a first group of monitors arrived on Monday in Syria on a month-long renewable mission to implement an Arab League peace plan.
The Britain-based Observatory reported on Thursday that security forces shot dead six people in Damascus province, six in the central city of Hama, five in Idlib province and four more in Homs.
"Security forces are raiding a private hospital in Hama and are arresting the wounded," it said.
"Huge protests" also took place in Hama's Hamidiyeh and Bab Qubli neighbourhoods, added the Observatory.
Emboldened by the presence of observers, Facebook activists urged regime opponents to take to the streets across Syria on Friday, now a traditional day of protest.
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"On Friday we will march to the squares of freedom, bare-chested," they said.
"We will march as we did in Homs and Hama where we carried olive branches only to be confronted by Bashar's gangs who struck us with artillery and machinegun fire," said the Syria Revolution 2011 activists.
The head of the Observatory, Rami Abdel Rahman, said protesters needed to make their voices heard to the monitors.
"The Arab League's initiative is the only ray of light that we now see," Abdel Rahman told AFP.
"The presence of the observers in Homs broke the barrier of fear."
On Tuesday, when a group of observers entered Homs on the first leg of their mission, some 70,000 people flooded the streets and were met with gunfire and tear gas, forcing the monitors to cut short their visit, activists said.
France, the United States and Human Rights Watch have warned the Syrian regime against trying to hide the facts from the monitors, and Paris charged that the team was not being allowed to see what was happening in Homs.
Those concerns were highlighted when Baba Amro residents on Wednesday refused to allow in observers in because they were accompanied by a Syrian army officer. The stand-off ended when the officer withdrew.
General Mohammed Ahmed Mustafa al-Dabi, a veteran Sudanese military intelligence officer who is heading the observer mission, has told AFP the visit to Homs was "good" and that Syrian authorities were cooperating so far.
His remarks reportedly triggered discontent among opposition ranks, but Abdel Rahman said it was too early to issue any judgment.
For some, Dabi is a controversial figure because he served under Sudan's President Omar al-Bashir who is wanted by the International Criminal Court for crimes allegedly committed in the Darfur region.
The League mission is part of an Arab plan endorsed by Syria after weeks of stalling. It calls for the withdrawal of armed forces from towns and residential districts, a halt to violence and the release of detainees.
Egypt's MENA news agency reported the head of the opposition Syrian National Council Burhan Ghaliun met Arab League Secretary General Nabil al-Arabi in Cairo ahead of a conference on Syria to be hosted by the League next month.