“The Arab ministerial committee expressed its rejection of the continued killings of civilians in Syria and expressed its hope that the Syrian government will take the necessary measures to protect them," Foreign ministers of the Arab League countries said in a statement published Friday.

Most of the latest killings took place in Hama, in the north, and Homs in the centre, the Syrian Observatory for Human Rights said.

Syrian security forces had encircled mosques to prevent protesters from demonstrating after weekly Muslim prayers, firing live rounds to disperse protesters, the Britain-based Observatory said.

They had killed at least 45 people, including a 15-year-old boy in Idlib province, wounded more than 100 and arrested 500.

Hama and Homs are at the front line of the anti-regime protests that have rocked Syria since mid-March, since when the UN estimates more than 3,000 people, mostly civilians, have been killed.

The army has been carrying out operations in Qusayr, near the Lebanese border, for several weeks, amid fighting there between troops and suspected army deserters, activists say.

The Observatory's chief Rami Abdel Rahman told AFP in Nicosia that Homs had seen the "highest number of martyrs to date" since the protest movement unfolded, accounting for 40 percent of protesters' deaths.

The latest violence was the deadliest in nearly six months to occur on a Friday, the day worshippers emerging from weekly prayers at mosques defy the security forces and swarm the streets to rally against the regime.

The worst day's violence was on April 22, when toll reached 72.

Each Friday protesters rally around a theme. This time they demanded the imposition of a no-fly zone to protect civilians and to encourage soldiers to defect -- like the UN-mandated no-fly zone over Libya that helped topple dictator Moamer Kadhafi.

"We call on the international community to impose a no-fly zone so that the Syrian Free Army can function with greater freedom," the Syrian Revolution 2011, one of the main movers behind the dissent, said on its Facebook page.

A defecting army officer who has taken refuge in Turkey, Colonel Riad al-Asaad, claimed in July to have established an opposition armed force called the "Syrian Free Army," but its strength and numbers are unknown.

On October 4, permanent UN Security Council members Russia and China vetoed a proposed resolution that would have threatened the Syrian leadership with "targeted measures" unless it halted the bloody repression.

According to the Observatory that rallying call was taken up by protesters in Kafr Nabl, in the northwestern province of Idlib bordering Turkey; and in Homs, where massive protests went ahead despite a security clampdown.

In the restive Balaa neighbourhood of Homs, around 20,000 people marched calling for the fall of President Assad's regime, the Observatory said.

But young protesters in the Barzeh district of Damascus were quickly overcome by the security forces, who arrested 40 demonstrators, the watchdog said.

In Qusayr meanwhile, where security forces sought to break up demonstrators streaming out of several mosques, activists reported hearing heavy gunfire and five explosions.

Suspected army defectors and troops reportedly also clashed in Hama.

Troops raided the northwestern town of Kafruma, arresting 13 people, including a woman and her 12-year-old son, while the funeral of an army deserter in Maaret al-Numan, turned into an anti-regime rally. Protesters in Deir Ezzor, further east, also came under fire.

In the United States, Syrian-born US citizen Mohamad Anas Haitham Soueid, 47, pleaded not guilty in court Friday to charges that he had spied on anti-Assad protesters for Syrian intelligence. He was remanded in custody for trial on March 5.

The Syrian embassy has vehemently denied the accusations, denouncing them as a "campaign of distortion and fabrications."

Spain meanwhile summoned Syria's ambassador over allegations that members of his embassy had abused Syrian opposition sympathisers on Spanish soil, the foreign ministry said.

Human rights watchdog Amnesty International on October 3 reported intimidation and torture of opposition sympathisers living in Europe and the United States, and of their relatives in Syria.