The assault in Homs, which has been one of the main flashpoints of opposition during the uprising, comes as the U.N. Security Council prepares to vote on a draft resolution backing an Arab call for President Bashar Assad to give up power.
Two main opposition groups, the British-based Syrian Observatory for Human Rights and the Local Coordination Committees, said the death toll in Homs was more than 200 people in shelling that began late Friday. More than half of the killings -- about 140 -- were reported in the Khaldiyeh neighborhood.
"This is the worst attack of the uprising, since the uprising began in March until now," said Rami Abdul-Rahman, the head of the Observatory, which tracks violence through contacts on the ground.
It was not immediately clear what precipitated the attack, but there have been reports that army defectors set up checkpoints in the area and were trying to consolidate control.
Earlier on Friday, deadly clashes erupted between government troops and rebels in suburbs of the Syrian capital and villages in the south, sparking fighting that killed at least 23 people, including nine soldiers, activists said.
Assad is trying to crush the revolt with a sweeping crackdown that has so far claimed thousands of lives, but neither the government nor the protesters are backing down and clashes between the military and an increasingly bold and armed opposition has meant many parts of the country have seen relentless violence.
Assad's regime has been intensifying an assault against army defectors and protesters. The U.N. said weeks ago that more than 5,400 people have been killed in violence since March. Hundreds more have been killed since that tally was announced.
The U.N. Security Council will meet this morning to take up a much-negotiated resolution on Syria, said a diplomat for a Western nation that sits on the council.
The move toward a vote came after Secretary of State Hillary Clinton spoke by telephone with Russian Foreign Minister Sergey Lavrov in an effort to overcome Russian opposition to any statement that explicitly calls for regime change or a military intervention in Syria.
The U.S. and its partners have ruled out military action but want the global body to endorse an Arab League plan that calls on Assad to hand power over to Syria's vice president.
Russia's deputy foreign minister, Gennady Gatilov, said Friday that Moscow could not support the resolution in its current form. But he expressed optimism that an agreement could be reached, according to state news agency RIA Novosti.
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