Up to 200 killed in protests in Libya, rights groups say (UPDATES) (VIDEO)

Libyan security forces have reportedly killed up to 200 protesters demanding the ousting of Muammar Gaddafi, in an attack labeled "horrifying" by the British foreign secretary.

But there are also reports of Libyan troops defecting to the side of the protesters: A witness told Al Jazeera that a section of the troops had joined the protesters on Sunday as chaos swept the streets of Benghazi, the worst-hit the city in the uprising against Gaddafi's 42-year old rule.

U.S.-based Human Rights Watch upgraded its death toll several times Sunday, saying most of those killed were involved in protests in the eastern cities of Benghazi, Bayda and Tobruk. The group called it one of the worst crackdowns so far in the wave of anti-government protests across the Middle East.

A group of 50 Libyan Muslim leaders on Sunday 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 said, Al Jazeera reported. "We appeal to every Muslim, within the regime or assisting it in any way, to recognize 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!"

A doctor in Benghazi told Al Jazeera that he had seen 70 bodies at the city's hospital on Friday. "I have seen it on my own eyes: At least 70 bodies at the hospital," said Wuwufaq al-Zuwail. He added that security forces had prevented ambulances from reaching the site of the protests.

Residents in Benghazi, about 600 miles east of the Libyan capital, Tripoli, reported Saturday that there was no electricity in parts of the city and that tanks were stationed outside the court building.

Human Rights Solidarity, a campaign group, said that snipers on rooftops in Al-Baida — a city of 210,000 — had killed 13 protesters and wounded dozens of others. Police stations in the town were set on fire as protesters burned posters of Gaddafi, according to the Telegraph.

British Foreign Secretary William Hague, meanwhile, said Saturday that he was "deeply concerned" by the "unacceptable violence" used against protesters in Libya, Bahrain and Yemen.

In a statement, Hague condemned the violence in Libya, saying reports of the use of heavy weapon fire and a unit of snipers were "horrifying." He called on the government to stop using force and rein in the army.

He said media access had been "severely restricted, and I have also received reports that 35 bodies were brought to one hospital alone."

Al Jazeera reported that the Libyan government had blocked its TV signal, and quoted those inside the country as saying that the network's website was inaccessible from there. Internet-based movements, inspired by activists inside Libya and abroad, have served as a rallying point for protesters.

One Libyan Facebook group, which had 4,400 members on Feb. 14, had seen that number more than double to 9,600 by Feb. 16, a day after clashes in Benghazi, Libya's second city.

Gaddafi has been in power in Libya since 1969. He reportedly used state television to berate citizens of neighboring Tunisia after their ouster of reviled president Zine al-Abedine Ben Ali.

Gaddafi's opponents say they, too, want political freedoms, respect for human rights and an end to corruption.

At the first sign of protest in Libya, the government proposed the doubling of government employees' salaries and released 110 suspected anti-government figures who oppose him — tactics similar to those adopted by other Arab governments facing recent mass protests. Gaddafi has also met with tribal leaders to solicit their support.

Saif al-Islam, the pro-reformist son of the dictator, has reportedly privately considered endorsing the protests in Egypt and elsewhere but was warned off doing so publicly by the security services.

Gaddafi's billionaire lifestyle and playboy reputation is a liability in a region where unemployment and corruption are provoking demonstrations, inspired by the ousting of long-serving rulers in Egypt and Tunisia.

But pro-Gaddafi supporters have also been vocal in their support of the dictator.

Meanwhile, violence was reported at anti-government protests elsewhere in the Middle East on Saturday.

In Yemen, police opened fire on demonstrators in front of Sanaa University on Saturday, marking the first time people were shot at during protests in Sanaa and the ninth consecutive day of violent clashes between pro- and anti-government supporters in the Yemeni capital.

In Bahrain, thousands of protesters poured back into Pearl Square on Saturday, singing and dancing in celebration following two straight days of a bloody crackdown.