|Libyan leader Muammar Gaddafi is confronting the most serious challenge to his rule in 42 years [Al Jazeera]
Security forces have shot dead scores of protesters in Libya's second largest city, where residents said a military unit had joined their cause.
While Libyan leader Muammar Gaddafi attempted to put down protests centred in the eastern city of Benghazi against his four-decade rule, Al Jazeera began receiving eyewitness reports of "disturbances" in the capital Tripoli early on Monday as well.
There were reports of clashes between anti-government protesters and Gaddafi supporters around the Green Square.
"We are in Tripoli, there are chants [directed at Gaddafi]: 'Where are you? Where are you? Come out if you're a man," a protester told Al Jazeera on the phone.
A resident told the Reuters news agency that he could hear gunshots in the streets and crowds of people.
"We're inside the house and the lights are out. There are gunshots in the street," the resident said by phone. "That's what I hear, gunshots and people. I can't go outside."
An expatriate worker living in the Libyan capital told Reuters: "Some anti-government demonstrators are gathering in the residential complexes. The police are dispersing them. I can also see burning cars."
There were also reports of protesters heading to Gaddafi's compound in the city of Al-Zawia near Tripoli, with the intention of burning the building down.
Meanwhile the head of the Al-Zuwayya tribe in eastern Libya has threatened to cut off oil exports unless authorities stop what he called the "oppression of protesters", the Warfala tribe, one of Libya's biggest, has reportedly joined the anti-Gaddafi protests.
Speaking to Al Jazeera, Shaikh Faraj al Zuway said: "We will stop oil exports to Western countries within 24 hours" if the violence did not stop. The tribe lives south of Benghazi, which has seen the worst of the deadly violence in recent days.
Akram Al-Warfalli, a leading figure in the Al Warfalla tribe, one of Libya's biggest, told the network: "We tell the brother (Gaddafi), well he's no longer a brother, we tell him to leave the country." The tribe lives south of Tripoli.
Protests have also reportedly broken out in other cities, including Bayda, Derna, Tobruk and Misrata - and anti-Gaddafi graffiti adorns the walls of several cities.
Anti-government protesters in the eastern Libyan city of Benghazi have reportedly seized army vehicles and weapons amid worsening turmoil in the African nation.
A local witness said that a section of the troops had joined the protesters on Sunday as chaos swept the streets of the city, worst hit by the uprising against Muammar Gaddafi's 42-year old rule.
Mohamed, a doctor from Al Jalaa hospital in Benghazi, confirmed to Al Jazeera that members of the military had sided with the protesters.
"We are still receiving serious injuries, I can confirm 13 deaths in our hospital. However, the good news is that people are cheering and celebrating outside after receiving news that the army is siding with the people," he said.
"But there is still a brigade that is against the demonstrators. For the past three days demonstrators have been shot at by this brigade, called Al-Sibyl brigade."
The witness reports came on a day in which local residents told Al Jazeera that at least 200 people had died in days of unrest in Benghazi alone. The New York-based Human Rights Watch on Sunday put the countrywide death toll at 173. The rights group said its figure was "conservative".
News of the rising death toll came as residents of Benghazi, Libya's second largest city, reported renewed gunfire from security forces in the city.
Sadiq al Ghiryani, a Libyan religious leader, told Al Jazeera a "massacre" was under way in the city and troops firing shots were mostly mercenaries.. Kamal Hudethifi, a judge, described the killings as "ethnic cleansing".
The Reuters news agency said at least 50 people had been killed in Benghazi since Sunday afternoon.
Moftah, a Benghazi resident , who requested Al Jazeera use only his first name, said the city had become a "war zone" in recent days.
Residents have barricaded the streets with overturned trash cans and debris, and security forces have largely confined themselves to two compounds, though snipers continue to target protesters, he said.
The forces who remain are "thugs" loyal to Gaddafi, Moftah said, and they are firing high-calibre ammunition at protesters.
The eyewitness report came a day after security forces opened fire at a funeral in the eastern coastal city on Saturday, killing at least 15 people and injuring scores more.
A group of six alleged mercenaries - reportedly brought in from Tunisia and other African nations to bolster pro-Gaddafi forces - were captured and arrested by demonstrators in the city of Shahat.
Appeal for calm
Against this backdrop of violence, opposition groups said some 50 Libyan Muslim leaders have urged security forces to stop killing civilians.
"This is an urgent appeal from religious scholars, intellectuals, and clan elders from Tripoli, Bani Walid, Zintan, Jadu, Msalata, Misrata, Zawiah, and other towns and villages of the western area," the appeal, signed by the group of leaders, stated.
"We appeal to every Muslim, within the regime or assisting it in any way, to recognise that the killing of innocent human beings is forbidden by our Creator and by His beloved prophet of compassion, peace be upon him ... Do not kill your brothers and sisters. Stop the massacre now!"
Around the world, people have been gathering in solidarity with the protesters at Libyan consulates and at the White House in Washington, DC, the US capital.
Libya's government has responded to the international criticism by threatening retaliation against the European Union. It said on Sunday that it would stop co-operating with efforts to try and stop illegal migrants heading to Europe.
Verifying news from Libya has been difficult since the protests began, because of restrictions on journalists entering the country, as well as internet and mobile phone blackouts imposed by the government.
The Libyan government has blocked Al Jazeera's TV signal in the country - and residents have also reported that the network's website is inaccessible from there.
This affects viewers on Arabsat and Nilesat at 26 degrees east and 7 degrees west, where alternative frequencies have now been set up.
A spokesman for the network said whoever was causing the interference must be using large outstations to simultaneously interfere with several platforms on the two orbital positions of Arabsat and Nilesat.
"We have set up alternative frequencies for viewers and are investigating the source of the problem, though cooperation would be needed from governments to precisely determine this," said the network.
"We believe that whoever is doing this is operating with sophisticated and large equipment."
In addition to TV signal jamming, internet service has been cut, said a US company that monitors web traffic.
Massachusetts-based Arbor Networks said data collected from 30 internet service providers worldwide showed that online traffic in and out of Libya was disconnected abruptly at 2:15am local time on Saturday. The data also showed two partial service interruptions earlier in the day.
As of Sunday, it was still possible to reach Libyans by phone, and some in Tripoli had internet access.