Syrian forces shot dead at least 40 civilians on Friday when they fired on demonstrators demanding international protection from President Bashar al-Assad's crackdown on pro-democracy protests, activists and residents said.
Tens of protesters were also wounded and hundreds arrested in one of the bloodiest days in seven months of protests demanding and end to 41 years of Assad family rule, prompting Arab ministers to send Assad their strongest message yet calling for an end to civilian killings.
The Arab League's committee on the Syrian crisis said on Friday it had sent an urgent message ... to the Syrian government expressing its severe discontent for the continued killing of Syrian civilians.
The committee said in a statement it had expressed the hope that the Syrian government would take action to protect civilians.
Arab ministers are due to meet Syrian officials on Sunday in the Qatari capital of Doha.
Most of Friday's killings occurred in the central cities of Hama, where Assad sent tanks and troops to crush large demonstrations three months ago, and Homs, a centre of protests and an increasingly armed opposition to his autocratic rule.
A no-fly zone is a legitimate demand for Homs, read banners carried by protesters in the Khalidiya neighbourhood.
NATO warplanes played a central role in the overthrow of Libyan leader Muammar Gaddafi, but the Western alliance has shown no appetite to intervene in Syria to halt violence which the United Nations says has killed 3,000 people.
Syria's opposition National Council has called for international protection. It has not explicitly requested military intervention, although street protesters have increasingly voiced that demand.
Assad has not used warplanes against protesters and a no-fly zone would have little impact on the crackdown unless -- as in the case of Libya -- pilots attacked his ground forces and military bases.
The anti-Assad protesters have been energised by Gaddafi's death last week. The demonstrations have spread to the countryside since tanks stormed several cities three months ago, forcing protesters to change their tactics of assembling in main squares and large, open spaces.
Authorities organised big pro-Assad demonstrations this week, with tens of thousands rallying in Damascus and the eastern town of Hasaka on Wednesday, and more pouring on to streets of the Mediterranean city of Latakia on Thursday. School children and public employees were ordered to attend.
Syria, a majority Sunni Muslim nation of 20 million people, is dominated by Assad's minority Alawite sect, an offshoot of Shi'ite Islam.
Aware of potentially seismic geopolitical implications if Assad were to fall, leaders of the mostly Sunni nations across the Arab Middle East have been cautious about criticising him as they struggle to deal with their own problems from the fallout of the Arab Spring.
A Sunni ascendancy in Syria could also affect Israel and play havoc with regional ties, with Assad strengthening an alliance built by his father three decades ago while also keeping to his father's policy of avoiding armed conflict with Israel along the occupied Golan Heights frontier since a 1974 U.S.-brokered cease-fire.
Assad held an inconclusive meeting on Wednesday with Arab ministers seeking to end the bloodshed by mediating a dialogue between him and his opponents and pushing for political reforms.
The Arab League had urged both sides to agree to a dialogue within two weeks -- a deadline that looms on Monday.
The authorities said they had major reservations about the proposal, while opposition figures said they could not sit down for talks unless there was a halt to the killing of protesters, disappearances and mass arrests.
Three days left, and we have 220 martyrs and counting, read a placard carried by protesters in the neighbourhood of Rankous on the edge of Damascus. Yes to dialogue -- after the downfall of the regime, said another in Homs.
After months of mostly peaceful protests, an armed insurgency has emerged, mainly in rural regions and in Homs, a city of one million, 140 km (85 miles) north of Damascus, where troops and pro-Assad militiamen have assaulted old neighbourhoods that have often seen demonstrations.
God, Syria -- We want a no-fly zone over it, shouted protesters in the Bab Tadmur neighbourhood of Homs.
In Hama, activists and one resident said Assad loyalists fired at a demonstration demanding Assad's ouster as soon at it broke out of Abdelrahman Bin Aouf mosque in al-Qusour district.
Syria has barred most foreign media, making it difficult to verify reports from activists and from authorities, who blame foreign-backed armed groups for the violence. The authorities say gunmen have killed 1,100 soldiers and police.
(Additional reporting by Dominic Evans and Mariam Karouny in Beirut; Editing by Alistair Lyon and Ralph Gowling)