Tens of thousands of Syrians protested on Monday in state-backed rallies against unprecedented economic sanctions imposed by the Arab League over President Bashar al-Assad's military crackdown on popular unrest now in its ninth month.

State television showed rallies "supporting national unity and rejecting foreign interference" in the capital Damascus and the second city of Aleppo. There were demonstrations also in the eastern cities of Deir al-Zor and Hasaka, the TV said.

The 22-member Arab League approved economic sanctions against Syria on Sunday, the toughest imposed against a member state, isolating Assad's government over violent repression which the United Nations says has killed 3,500 people.

Britain said the sanctions could help enlist support at the United Nations for action against Damascus.

Anti-Assad activists said security forces had killed at least 24 civilians on Sunday, many in a town north of Damascus that has become a focus for the protests. Others were killed in raids on towns in the province of Homs.

"The indications are not positive ... the sanctions are still economic but if there is no movement on the part of Syria then we have a responsibility as human beings to stop the killings," Sheikh Hamad bin Jassim al-Thani, Qatar's prime minister and foreign minister, told reporters.

"Power is not worth anything when a ruler kills his people," he said after 19 of the League's 22 members meeting in Cairo on Sunday approved a decision to immediately enforce sanctions.

The measures include a travel ban on top Syrian officials and a freeze on assets related to Assad's government. They are aimed at halting dealings with Syria's central bank and investment in the country, Sheikh Hamad said.

He added that Turkey, which attended the Arab League meeting, would also honour some of the measures, dealing a further blow to a Syrian economy already reeling from sanctions imposed by the European Union and United States.

Arab nations wanted to avert a repeat of what happened in Libya, where a UN Security Council resolution led to Nato air strikes.

Sheikh Hamad warned Arab states that the West could intervene in Syria if it felt the League was not "serious".

British Foreign Secretary William Hague said the decision to impose sanctions against an Arab state was unprecedented.

It "demonstrates that the regime's repeated failure to deliver on its promises will not be ignored and that those who perpetrate these appalling abuses will be held to account," he said.

Hague said Britain hoped the move would help break what he called United Nations silence "on the ongoing brutality taking place in Syria", after Russia and China thwarted Western efforts to pass a UN Security Council resolution on Syria.

Hague welcomed "the commitment by the Arab League to engage with the UN Secretary General at the earliest opportunity to gain the UN's support to address the situation in Syria".

Britain has repeatedly ruled out a military attack on Syria. The sanctions represented the toughest Arab League action taken against a member state since the expulsion of Egypt in 1978 over its peace accord with Israel.

Cairo was later readmitted to the League. Assad, who inherited power from his father in 2000, said in an interview this month that he would continue the crackdown and blamed the unrest on outside pressure to "subjugate Syria".

Many Arab leaders have become increasingly concerned by a series of "Arab Spring" revolts that have toppled the rulers of Tunisia, Egypt and Libya.

One Western diplomat said Assad could for now count on support from China and Russia at the United Nations but they may change position if he intensifies the crackdown and if the Arab League campaigns for international intervention.

China and Russia have oil concessions in Syria. Moscow also has a little-used naval base in the country and provides military advisers to the Syrian army.

"The sanctions are likely to lose Assad support among those in Syria who have been waiting to see whether he will be able to turn things around, such as merchants who could now see their businesses take more hits," the diplomat said.

The president of the Union of Arab Banks, a division of the Arab League, said the sanctions would hit Syria's central bank, which has "big deposits" in the region, especially the Gulf.

Arab ministers were spurred to action by worsening violence in Syria and by the Assad government's failure to meet a deadline to admit Arab League observers and take other steps to end its crackdown on the uprising.

Syrian official media quoted an undated letter by Syrian Foreign Minister Walid al-Moualem to the Arab League as saying Damascus viewed the plan for outside monitors as interference.

The League has been galvanized by pressure from Gulf Arabs, already angry at Syria's alliance with regional rival Iran, by the political changes brought about by Arab uprisings, and by the scale of the Syrian bloodshed.

Along with peaceful protests, some of Assad's opponents are fighting back. Army defectors are grouped loosely under the banner of a Syrian Free Army and more insurgent attacks on loyalist troops have been reported in the last several weeks.

Syrian officials blame the violence on armed groups targeting civilians. Government security forces say 1,100 of their members have been killed.-Reuters