Syrian army defectors have attacked a major military base near Damascus, Syrian opposition groups say.
Parts of the notorious Air Force Intelligence building in Harasta were reported to have been destroyed, but there were no reports of casualties.
It would be the Free Syrian Army's (FSA) most high-profile attack since Syria's anti-government protests began.
The attack came ahead of an Arab League emergency meeting in Morocco to discuss ways of ending the bloodshed.
The meeting is expected to ratify last week's vote to suspend Syria and also debate possible further measures against Damascus.
Turkey, which is not a member but is attending the meeting, has said Syria will face "isolation" for its actions.
In a further sign of Syria's increasing isolation, France withdrew its ambassador on Wednesday.
Foreign Minister Alain Juppe told parliament: "There has been renewed violence in Syria, which has led me to close our consular offices in Aleppo and in Latakia as well as our cultural institutes and to recall our ambassador to Paris."
The UN says more than 3,500 people have died since protests started in March. Syrian authorities blame the violence on armed gangs and militants.
The Syrian government has severely restricted access for foreign journalists, and reports of violence are difficult to verify.
Several thousand people attended rallies in Damascus and Latakia on Wednesday in support of President Bashar al-Assad.
The rallies marked the anniversary of Mr Assad's father, Hafez al-Assad, seizing power in 1970.
The Syrian National Council (SNC) - a coalition of opposition groups based in Turkey - said the attack on the Harasta base had been carried out by the FSA.
Such an attack would be significant because Syria's Air Force Intelligence is one of the most feared state agencies and has been involved in the suppression of protests.
Activists said the defectors attacked the building from three sides. Helicopters - most likely government-operated - were reported to have been hovering around the area.
A resident of Harasta told Reuters: "I heard several explosions, the sound of machine-gun fire being exchanged."
The Local Co-ordination Committees also reported the attack, while the UK-based Syrian Observatory for Human Rights said there had been other assaults in Zamalke, Hamuriya and Douma.
The Khaled ibn Walid Brigade branch of the FSA - based in the flashpoint city of Homs - welcomed the attack in a statement, AFP news agency reports.
"We pay tribute to our brothers, the rebel heroes, and may God bless your hands for your dawn operation targeting the intelligence building in Harasta," it said.
The FSA was formed by defectors a few months ago, and claimed to have 15,000 members by mid-October, but this is widely considered an overstatement.
Its commander, Riyad al-Asad, has been in Turkey for the past few months.
The BBC's Jonathan Head, in Turkey, says his return, coupled with the increased reports of attacks against government troops, suggest the FSA is carrying out a determined assault on the military.
On Tuesday, the FSA was reported to have carried out an ambush in Deraa that left 34 government soldiers dead, according to Syrian activists.
The SNC has said it wants the anti-government movement to remain essentially peaceful, our correspondent adds, but an increasing number of defected soldiers appear ready to take up arms.
November appears to be the bloodiest month of the eight-month revolt, with more than 300 people killed so far.
The Syrian Observatory for Human Rights said on its Facebook page that three defectors and a civilian were shot dead in Keferzita, Hama province, on Wednesday. One person died in Hara in Deraa province and five people in the city of Homs, it said.
Mr Assad is coming under increasing international pressure to end the bloodshed, after failing to honour an Arab League peace plan which would have seen him step down in exchange for amnesty.
As Arab League foreign ministers gathered in Rabat on Wednesday, Turkish Foreign Minister Ahmet Davutoglu said Syria had failed to deliver on its promises.
"The cost for the Syrian administration of not fulfilling the promises it made to the Arab League is its isolation in the Arab World," he said.
Qatari Foreign Minister Hamad bin Jassim said the situation was "very sad" and called on Syria to abide by the Arab League plan.
Syria, which will not attend the meeting, has condemned the suspension as "shameful and malicious", accusing other Arab countries of conspiring with the West to undermine the regime.
In one of the most prominent criticisms so far, Jordan's King Abdullah said on Monday that if he were in Mr Assad's position he would step down.
"Whenever you exert violence on your own people it is never going to end well," he told the BBC.