Updated

A wave of violence killed 33 people in the last 24 hours in Pakistan's largest city, with many of the victims tortured, shot and stuffed in sacks that were dumped on the streets, officials said Thursday.

There has been a surge in killings in Karachi in recent weeks that many blame on gangs affiliated with the city's main political parties. The government has been unable to stem the violence, as it also grapples with a faltering economy and a raging Islamist insurgency.

The unrest illustrates the precarious state of Pakistan's stability at a time when the U.S. wants the nuclear-armed country to step up its fight against Taliban militants who stage cross-border attacks against foreign troops in Afghanistan.

Authorities were investigating the circumstances surrounding the 33 people killed in Karachi in the last 24 hours, said Saud Mirza, police chief in the teeming metropolis of some 18 million people. Many of the victims were tortured, shot in the head and stuffed in burlap sacks, he said.

A resident in one of the neighborhoods that has experienced much of the violence said people were afraid to leave their homes for fear of being killed. He spoke on condition of anonymity because of fear of being targeted.

The latest round of violence seemed to be driven by a mix of political and criminal motivations, said Sharfuddin Memon, the security adviser to the government in Sindh province, where Karachi is the capital.

"Gangs operating in the city are involved in the fresh killing," said Memon. "They are kidnapping people for different reasons, torturing and killing."

A former national lawmaker from the ruling Pakistan People's Party, Waja Kareem Dad, was gunned down Wednesday evening, said Memon. Others were also killed by gunfire and grenade attacks, he said.

Karachi has a long history of political, ethnic and sectarian violence, but the recent wave is high by historical standards. More than 300 people were killed in July alone.

The recent bout of violence followed a decision in late June by the Muttahida Qaumi Movement, the city's most powerful political party, to leave the federal coalition led by the Pakistan People's Party and join the opposition.

Fighting intensified in mid-July after a senior member of the People's Party, lashed out at the head of the MQM, calling him a murderer and an extortionist and maligning the city's Urdu-speaking community that makes up the party's base. He later apologized, but the fighting has continued.

The MQM dominates politics in urban areas of Sindh, including Karachi, but over time it has seen challenges to its power from the People's Party and the Awami National Party, a Pashtun nationalist party.

There were at least 490 political, ethnic and sectarian killings in Karachi during the first half of the year, and more than 1,100 killings of all kinds, according to the Human Rights Commission of Pakistan.