Anti-government rallies have broken out in the Libyan capital Tripoli for the first time during days of protests against the country's leader, Muammar Gaddafi.
The sound of gunfire can be heard across the city.
Witnesses say tear gas and live ammunition has been used by the security forces.
There are unconfirmed reports of clashes between pro- and anti-Gaddafi protesters in central Green Square.
Reports from the eastern city of Benghazi say more than 200 people have died there after the Libyan military used heavy weapons against protesters.
The brutal suppression of protests in Benghazi has been condemned by the US, UK and other Western nations.
In Tripoli, protests were reported in four districts of the city late on Sunday.
Gunfire rang out through central areas of Tripoli, accompanied by car horns blaring and people chanting slogans calling for the toppling of the regime.
Credible reports suggested that a police station had been burnt down and a building belonging to the country's ruling party had also been attacked and set ablaze.
In the Gourghi area of the city, witnesses said tear gas and live ammunition were used against anti-government protesters. Tear gas was said to have filled the entire neighbourhood, with those sitting outside their homes forced to to go inside.
The area around Tripoli's largest university was said to be dominated by anti-government protesters calling for the fall of the regime.
Correspondents say the spreading of unrest to Tripoli is a significant blow to Col Gaddafi's attempt to contain the protests in eastern Libya.
Protesters have been demanding an end to Col Gaddafi's four decades in power, after the success of similar rallies in Egypt and Tunisia.
By Sunday evening, reports suggested that anti-government forces in Benghazi had largely won control of the city after two days of brutal suppression by government forces.
Unconfirmed reports said a military garrison there had either fallen to the opposition, or switched sides.
Earlier, in a rare public admission of the unrest in Benghazi, Libyan state TV said on Sunday that an "armed people's base" in the city had come under attack and had its walls breached.
The US state department said it was "gravely concerned with disturbing reports and images coming out of Libya".
France said it was "extremely worried" by events in Libya, criticising Tripoli for a "totally disproportionate" use of force.
But Libya has also tried to use its leverage with the European Union, warning Brussels it could suspend co-operation against illegal migration if the EU encourages pro-democracy protests.
During a phone conversation with Colonel Gaddafi's son, Sayf al-Islam, UK Foreign Secretary William Hague spoke of London's "grave concern" at the escalation of violence.
Verifying information from Libya has been difficult amid a government clampdown on the internet and media communications. Libyan authorities have imposed severe media restrictions.
Foreign media are largely absent from the country and internet traffic from Libya has been throttled in recent days, web analysts report.
Libyan activists opposed to Col Gaddafi, as well as Libyans operating from outside the country, are using social networks Twitter and Facebook and video-sharing site YouTube to share information and images of the protests.
Libya is one of several countries in the region to have seen pro-democracy campaigns since the fall of long-time Tunisian President Zine al-Abidine Ben Ali in January. Egypt's Hosni Mubarak was forced from power on 11 February.