Al Jazeera says its signal is being jammed and its website has been blocked in Libya
Human rights group Amnesty International said on Friday its sources had said Libyan security forces had shot dead at least 46 people in the past three days.
Amnesty said in a statement sources at al Jala hospital in Benghazi had reported 28 deaths and more than 110 people injured in Thursday's protests in the city, and at least three further deaths on Friday.
Local human rights activists reported at least 15 deaths on Thursday during protests in the nearby town of Al Bayda, an Amnesty International spokeswoman said.
"This alarming rise in the death toll, and the reported nature of the victims' injuries strongly suggests that security forces are permitted use lethal force against unarmed protesters calling for political change," said Malcolm Smart, Amnesty International's director for the Middle East and North Africa.
"The Libyan authorities must immediately rein in their security forces. Those responsible for unlawful killings and excessive force — both the direct perpetrators and those who gave the orders — must be identified and brought to justice," he said in the statement.
Amnesty said the sources at al Jala hospital in Benghazi had told it the most common injuries were bullet wounds to the head, chest and neck.
Thousands of people protested in Benghazi, Libya's second city, on Friday over a security crackdown in which dozens of people were killed but which failed to halt the worst unrest of Muammar Gaddafi's four decades in power.
The Qatar-based news channel Al Jazeera had, meanwhile, said on Friday its signal was being jammed on several frequencies and its website had been blocked in Libya.
Al Jazeera, whose coverage of the political unrest in the Middle East and North Africa is widely watched in the Arab world, reported the jamming on its website where it offered alternative frequencies on the Arabsat, Nilesat and Hot Bird satellites.
Al Jazeera has closely followed events in Libya, Bahrain and Yemen, contacting protesters and government backers by telephone and often using footage of events sent via the Internet.
Bahraini security forces fired on protesters on Friday, wounding dozens, and thousands of people demonstrated in Libya after a deadly government crackdown as pro-democracy unrest in the Middle East and North Africa turned increasingly violent.
Earlier this month, Al Jazeera complained of similar jamming and Egypt's Nilesat cut off its signal for more than a week after the Egyptian authorities ordered the channel to stop operations in the country during the unrest that ended President Hosni Mubarak's 30-year rule.