KARACHI, Pakistan (AP) — The ruling party in Pakistan's largest city accused its main political rival of supporting Islamist militants suspected of assassinating a party leader, further stoking tensions Tuesday after 45 people died in a night of revenge attacks and arson.

The accusation appeared to reflect the complex and vicious political and ethnic faultlines that crisscross Karachi, also Pakistan's commercial hub and home to the main port for supplies to U.S. and NATO troops across the border in Afghanistan.

It has long been plagued by political violence between supporters of rival parties that draw votes from different ethnic groups that live in the city of 16 million people. Their supporters are accused of running protection rackets and illegally seizing land, muddying the picture as to the reasons for the bloodshed.

The killing of politician Raza Haider was the most high-profile in a series of slayings of party activists over the last month.

Pakistan's Interior Minister Rehman Malik was quoted by militants as saying Islamist militants were likely suspects, saying Haider was on an extremist hit list.

Haider was a senior member of the Muttahida Qaumi Movement, the party that runs Karachi and represents mainly descendants of Urdu-speaking migrants from India who settled in Pakistan when it was created in 1947. The MQM's main rival is the Awami National Party, a nationalist party made up mostly of ethnic Pashtuns from the northwest, where the Taliban are based.

Pashtuns have been arriving in the city in greater numbers in recent years, fleeing Pakistan army offensives against the Taliban. The MQM has long spoken out against the alleged "Talibanization" of the city. While some militants have found safe haven here, critics say the MQM is exploiting the issue for political purposes.

"If you see the record of militants arrests in past years you would see almost all of them were in the Pashtun-dominated areas," Faisal Sabzwari, an MQM leader, told The Associated Press. "And for sure the ANP gives them support." The charge was repeated in a televised news conference by other MQM leaders.

A leading ANP politician dismissed the allegation, noting the party has been repeatedly attacked by the Taliban in the northwest, where it supports the army offensives against them. "We share their (the MQM's) pain," said Mian Iftikhar Hussain, whose son was killed by insurgents last week.

Police promised to investigate the slaying, but few killers in such cases have ever been brought to justice, nor the motives for the attacks revealed.

Within hours of Haider's assassination, rival gangs torched buildings in Karachi and gunfire erupted in several parts of the city. Many of the dead were killed in targeted, execution-style attacks, authorities said, without revealing details on who they were.

Schools were closed and most business stopped Tuesday as the city braced for further violence, but by nightfall there were no reports of fresh unrest.

The linking of the ANP — and by extension the city's Pashtun population — to the killing of Haider risks opening fresh fissures. Authorities are normally very cautious in what they say to the media follow political slayings out of fear of triggered revenge killings.

In the 1980s and 1990s, the city was regularly convulsed by violence in which hundreds were killed. MQM leader Altaf Husein fled to London in 1992 as a result of that bloodshed and was granted asylum. He regularly addresses large gatherings of supporters by telephone link.

An estimated 4 million Pashtuns are in Karachi, more than in the main northwestern city of Peshawar.

Many live in sprawling slums on the outskirts that are "no-go" areas for authorities.

Sharfuddin Memon, the head of the Citizens' Police Liaison Committee, speculated the killing of Haider and others may be related to a government operation against so-called "land mafias", which illegally occupy commercially valuable land often allegedly with the backing of political parties.

"The assassination and the ensuing killing have diverted the government focus from that operation," said Memon. "And this mafia, it is not operating in isolation, there are elements in the civil administration who are in connivance with them," he said.