BENGHAZI, LIBYA - Government paramilitary forces opened fire Friday on protesters who swarmed the streets of Tripoli in what opponents hoped would be a final push to topple Moammar Gaddafi's regime. Witnesses described multiple casualties from the fiercest violence yet in the Libyan capital.

It appeared that the regime had retained control, for now, of its major remaining stronghold. After the clashes, a defiant Gaddafi urged thousands of his supporters at a rally in the heart of the city to take up arms on his behalf.

Yet even as the Libyan leader spoke, his 41-year grip on power seemed to loosen further. There were reports that rebels had gained control of at least one key suburb of Tripoli, and several other towns, including heavily contested Zawiya, 20 miles west of the capital, were said to have fallen to the opposition.

High-level defections continued to weaken Gaddafi's regime, and the world community stiffened its response. The United States said it would impose sanctions, and the United Nations advanced a process that could lead to a war crimes prosecution.

Some Tripoli residents expressed fears of a prolonged siege in which rebels control towns and cities around the country while Gaddafi's forces in Tripoli dig in.

"We know the whole country is with us, but we don't know how long this is going to take," said a trader who joined the protests but went home after the gunfire became too intense. "The security forces have the upper hand, and there's so many of them, because he's concentrating all his effort on Tripoli."

While Gaddafi and his remaining allies - including his son, Saif al-Islam Gaddafi - promised a fight to the death, much of the country has celebrated the spreading collapse of the regime, with some taking steps to support protesters in the capital.

The day's fighting in Tripoli began shortly after midday prayers, when residents poured out of mosques to revive their protests, only to be met by gunfire from soldiers and armed pro-Gaddafi civilians.

For several hours, the protesters pressed ahead in their attempt to converge on the city's central Green Square, chanting "God is great" and anti-Gaddafi slogans. Videos posted on YouTube and Facebook showed scenes of citizens scattering under volleys of fire in several neighborhoods, then attempting to regroup.

But by nightfall the pro-government forces, including militias and paramilitary forces, appeared to gain the upper hand. Most protesters retreated indoors, witnesses said.

It was Gaddafi, wearing a Russian-style fur hat, who assembled thousands of cheering supporters in the square. Standing on the ramparts of a castle and shaking his fist, he vowed to open state arsenals and distribute weapons to protect his regime.

"Every Libyan individual will be armed, every Libyan tribe will be armed. So Libya will turn to hell," he said, the square packed with people waving green flags.