(CNN) -- Somalia's main opposition group Monday accused Ethiopian troops of killing six leaders of a Muslim sect during weekend street battles in Mogadishu, clashes one observer called the worst since the country's government collapsed in 1991.
A victim of the fighting receives medical treatment south of Mogadishu, Somalia, on Sunday.
Sheikh Saiid Yahye and five other leaders of the Tabliiq, a Muslim missionary group unaffiliated with Somalia's warring factions, were killed Sunday when Ethiopian troops seized their mosque in northeastern Mogadishu.
Elmi Abdulqadir, a local elder who helped bury the dead, said the bodies were left outside the mosque with their arms bound and their throats cut.
The killings provoked widespread condemnation in the Somali capital, where bodies were strewn on the streets Monday after two days of fighting between government and Ethiopian troops and Islamist insurgents that left dozens dead.
Mogadishu-based Elman Human Rights Group reported at least 81 civilians were killed and more than 100 were wounded as government and Ethiopian troops battled Islamic militants in Somalia's capital over the weekend. See scenes from the fighting »
"This is utterly a violation of the international law," said Hassan Muse Olow, a university student who spoke to CNN. "The killing of these clerics happened in a holy place, and it now looks that the war in Mogadishu is losing all limits."
Somali government officials did not respond to requests for comment.
The country's main opposition front, the Alliance for the Re-Liberation of Somalia, threatened to boycott peace talks set for May after the weekend deaths, which its leader called a "massacre" by the Ethiopians.
"The Ethiopian forces backing the transitional government mercilessly killed many innocent people, including the people slaughtered at the al-Hidaya mosque," Sheikh Sharif Ahmed, the group's leader, said Monday. "I am clearly stating that we will rethink about attending the expected talks in Djibouti."
Ahmed is the former chairman of the Islamic Courts Union, which took control of Mogadishu in 2006 only to be pushed out by Ethiopian troops six months later. The Alliance for the Re-Liberation of Somalia includes former ICU leaders, Somali exiles and former members of the country's U.N.-backed transitional parliament.
Somalia has been mired in chaos since 1991, when warlords overthrew dictator Mohamed Siad Barre and sparked brutal clan infighting. Ethiopia invaded in December 2006 to install the transitional government in Mogadishu -- but their presence has united various Islamic militant groups in Somalia, who launched a guerrilla war that has lasted for more than a year.
"The level of violence is very high in Somalia, but in particular Mogadishu is going through its worst time in terms of war for the last 17 years of the conflict," Abdullahi Alas, of the Dr. Ismail Jumale Center for Human Rights in Mogadishu, told CNN. "Deaths of civilians are just going up each day, and there is also an unspeakable displacement of civilians from the capital."
New fighting hit the city's Bakara market district Monday, killing at least two more civilians, witnesses said.
At the United Nations, Secretary-General Ban Ki-moon expressed concern over the renewed violence and the "substantial loss of life" among civilians.
"He urges parties to the conflict in Mogadishu to refrain from the indiscriminate and disproportionate use of force that endangers the lives of civilians, particularly in heavily populated civilian areas, and reminds them that any targeting of non-combatants is a violation of international humanitarian law," the secretary-general's office said.
Sudan Ali Ahmed, the head of Elman Human Rights Group, accused both sides of war crimes Sunday. He said the rebels were using civilians as human shields, while Ethiopians unleashed tanks and artillery on residential neighborhoods.
"The Ethiopians are firing heavy weaponry into areas where civilians are heavily populated, while the Islamic militants are firing mortars and RPGs from within those areas," Ahmed said. "So the exchange is causing a civilian casualty toll which is unspeakable."
Witnesses reported large numbers of people fleeing Mogadishu during the weekend's battles, joining a population of displaced Somalis that aid groups estimate already tops 1 million.
Monday morning, Somali security forces arrested a prominent journalist as he headed into work in Mogadishu. No reason for his arrest was disclosed.
A source at Shabelle Media Network, the privately funded organization where the news editor known as Abdi Uud works, said he suspects Uud was arrested for Shabelle's reporting on the recent fighting. The National Union of Somali Journalists issued a statement condemning the arrest of Uud, whose real name is Abdi Mohammed Ismail. E-mail to a friend