Messages and videos posted on social media sites Thursday signaled that anti-government protests in Libya were gathering steam in several cities, with some turning violent on a "Day of Rage."
There were reports of 16 deaths, but they could not be independently confirmed.
A text message sent out earlier on mobile phones had threatened Libyans planning to take to the streets, activists and bloggers said.
"From Libya's youth to anyone who dares to cross any of the four red lines come and face us in any street on the ground of our beloved country," the Short Message System dispatch said, referring to a speech by Saif el-Islam Gadhafi, Moammar Gadhafi's son, in which he described the lines as Islamic law, the Quran, Libyan security and his father.
They apparently did little to deter demonstrators. Protests in the isolated North African nation broke out this week, part of a larger anti-government movement sweeping the region.
CNN does not have journalists in Libya and was unable to confirm the extent of the demonstrations unfolding there.
Human rights groups have reported gradual moves toward freedom of expression in Libya, but Gadhafi's government retains control over most of the media in Libya and monitors and censors the fledgling private media outlets.
Libyan websites had posted calls for a "Day of Rage" on Thursday, the anniversary of a 2006 demonstration in which security protests killed at least 12 protesters.
State-run television, however, showed demonstrations in support of Gadhafi, who has ruled Libya for more than four decades.
A source in Tripoli who is close to senior members of the government told CNN that several events Thursday showed support for Gadhafi, including a gathering of several hundred people in the capital and others riding, brandishing Libyan flags from their cars.
The source had said Wednesday that he was not aware of further anti-government protests following a demonstration in Benghazi Tuesday night.
But Ahmed Elgasir, a researcher at the Geneva, Switzerland-based Libyan Human Rights Solidarity, said that serious clashes between protesters and security forces occurred Thursday in Benghazi.
The day before, at least 38 people were injured after police fired tear gas and used batons to disperse crowds in Benghazi, the online newspaper Quryna reported.
Human Rights Watch reported one death in Benghazi -- based on its sources inside the country who were not identified because of security concerns.
Enough Gaddafi, an anti-Gadhafi Libyan exile group in the United States, reported six deaths in Beghazi, based on local sources.
Elgasir said his group's sources on the ground said 10 people were killed in Baida Thursday and that the city was surrounded by security forces. He said he was unable to contact people in Baida Thursday because the internet and cell phone text messaging had been shut down.
Abdulla Darrat, spokesman for Enough Gaddafi, told CNN that hospitals in Baida were inundated and that doctors were running out of medical supplies to treat the injured.
Elgasir also said demonstrations were taking place in Zentan, south of Tripoli.
Gadhafi is acutely aware of discontent with the government and has been moving to address popular grievances before they surfaced on the streets, an independent source told CNN.
Libya, like many of its Arab neighbors, is suffering from economic hardship and a lack of political reform. Unemployment rates among the nation's youth are high.
Gadhafi has spoken with groups of students, lawyers and journalists in the past few weeks, the source told CNN.
On one occasion, the longtime ruler appointed an outspoken member of the lawyers' union as its leader, removing the syndicate's old guard, according to the source. He also promised action to address the country's housing crisis.
Human Rights Watch reported that security forces have arrested 14 Libyans in connection with the demonstrations. Among them were human rights activists, journalists, lawyers and two key members of the families of an alleged 1996 massacre at the Abu Salim jail.
Demonstrations erupted after the detention of the family members, the rights group said. There have been regular public protests for the past two years calling for an independent probe into the killings at Abu Salim.
Novelist Idris al-Mismari told the al-Jazeera network that plainclothes security officers used tear gas, batons and hot water on the crowds. During his live interview, the line went dead. Witnesses told Human Rights Watch that he was arrested then.
One of the protesters likened the situation in Libya to Egypt, telling Human Rights Watch that "they are sending baltaqiyyas (thugs) to beat us."
U.S. State Department spokesman P.J. Crowley said Libya should listen to its people.
Secretary of State Hilary Clinton had made clear that "countries across the region have the same kind of challenge in terms of the demographics, the aspirations of their people, the need for reform," he said.
"And we encourage these countries to take specific actions that address the aspirations and the needs and hopes of their people," Crowley said in a news briefing Wednesday. "Libya certainly would be in that same category."
A highly placed Libyan source close to the government sought to downplay the reports of unrest. The source asked not to be identified because he was not authorized to talk to the media.
"There is nothing serious here," the source said Wednesday. "These are just young people fighting each other."