BEIRUT -- Syrian rebels fired grenades at tanks and troops while regime forces shelled Damascus neighborhoods on Monday, sending terrified families fleeing the most sustained and widespread fighting in the capital since the start of the uprising 16 months ago.

A ring of fierce clashes nearly encircled the heavily guarded capital as rebels seeking to overthrow President Bashar Assad pushed the civil war that has been building in Syria's impoverished provinces closer to the seat of power.

While the clashes were focused in the city's southwest, for many of its 4 million people the violence brought scarily close to home the strife that has deeply scarred other Syrian cities.

In high-end downtown cafes frequented by the business and government elite tightly bound to the Assad regime, customers watched as black smoke billowed on the horizon and the boom of government shells reverberated in the distance.

On Monday, Russian Foreign Minister Sergey Lavrov accused the West of using blackmail to secure a U.N. Security Council resolution that could allow the use of force in Syria.

Lavrov objected to the text of a West-backed resolution that calls for sanctions and invokes Chapter 7 of the U.N. Charter, which can be enforceable militarily.

He said Russia had been told that if it opposed the resolution, Western nations would not extend the mandate of a U.N. mission sent to Syria to monitor a cease-fire.

"We consider it to be an absolutely counterproductive and dangerous approach," Lavrov said.

Monday's fighting suggested that deep cracks were appearing in the tightly controlled facade of calm that has insulated Damascus from violence throughout the uprising.

Damascus -- and Syria's largest city, Aleppo -- are both home to elites who have benefited from close ties to Assad's regime, as well as merchant classes and minority groups who worry their status will suffer if Assad falls.

But for months, rebels have been gaining strength in poorer towns and cities around Damascus. Some activists suggested Monday that recent government crackdowns in those areas had pushed rebels into the city, where they were determined to strike at the heart of the regime.

"It seems there is a new strategy to bring the fighting into the center of the capital," said activist Mustafa Osso. "The capital used to be safe. This will trouble the regime."

Another activist, who gave only his first name, Moaz, said he had never seen such violent fighting in his neighborhood of Tadamon, a poor, densely populated area south of downtown.

He said the army had parked armored vehicles at the neighborhood's entrances and posted tanks on its north and south edges.

Some two-thirds of the neighborhood's residents have fled; those who remain are scared government snipers will target them if they leave now, he said.

But so far, the rebels have kept the army out, destroying three tanks and one armored car with rocket-propelled grenades, said Moaz, declining to give his full name for fear of retribution. Others spoke on condition of anonymity.

The government said little about the clashes, but the state news agency said the army was hunting an "armed terrorist group" in one of the neighborhoods.

Streets were largely deserted in neighborhoods near the fighting. Many families have fled or are still trying to get out, and fear grips those who remain.

"It is a war here, a war," said a 28-year-old mother of two reached by phone in the Midan neighborhood. She said she didn't know if there were rebels on her street because she was scared that looking out the window would draw fire. She said her 5-year-old son had not stopped screaming since the fighting started Sunday.

On Monday, Morocco asked the Syrian ambassador to leave the country.