Pakistan's leading human rights organisation on Wednesday accused the government of failing to stop political killings after officials said another night of violence left 24 people dead in Karachi.

The Human Rights Commission of Pakistan (HRCP) says 1,300 killings have been reported in the country's biggest city this year, 490 of whom were victims of targeted killings blamed on political and ethnic tensions.

It blamed the government, led by the Pakistan People's Party (PPP) of President Asif Ali Zardari, over the killings, which underscore deep insecurity in the country's economic hub used by NATO to ship supplies to Afghanistan.

The political violence has been blamed on loyalists of ex-coalition partners the Muttahida Qaumi Movement (MQM) and Awami National Party (ANP), which represent different ethnic communities and straddle volatile fault lines.

"We have got reports that at least 24 people have fallen victim to targeted killings since Tuesday evening," Sharfuddin Memon, an official of the home ministry in the southern province of Sindh, told AFP.

He said the police had arrested 12 men with guns and ammunition for alleged involvement in the killings.

A police surgeon, Hamid Parhiar, said city hospitals had received the bodies of 15 victims.

A security official said tensions in the city's western Orangi and Qasba neighbourhoods, where gunmen were still opening fire intermittently, had left streets deserted and markets closed.

"Police and paramilitary troops are on patrol in the troubled neighbourhoods to avert further violence," Memon said.

The police said five of the bodies were found on a bus in the eastern neighbourhood of Gulshan-e-Iqbal.

"We saw a bus parked suspiciously along a street and found bodies of five people -- all shot dead -- inside. Their identities are still unknown," police official Mohammad Hashim said.

Both MQM and ANP say the dead include their supporters.

"We want the government to tackle this issue on objective grounds, strengthen and de-politicise the police and de-weaponise the city. Every party in Karachi has weapons," Zohra Yusuf, HRCP chairman, told AFP.

"Many police officials have complained that they receive calls from the ruling quarters for the release of suspects of targeted killings soon after their arrests," she said.

"This shows how deep these killings are rooted inside the political parties."

HRCP said 490 people had been victims of killings linked to politics in the first half of the year, compared to 748 people in all of 2010 and 272 in 2009.

Police said they had recovered 90 Russian-made hand grenades hidden in a metal barrel from a sparsely populated Sachal suburb of the city.

"We believe terrorists put them here to use in future terror acts on the city," police official Ahsan Umer told AFP.

Karachi has witnessed several recent attacks in which hand grenades were hurled in public places, including one that injured 17 people in a southern suburb on June 27.

Karachi is also plagued by sectarian killings, crime and kidnappings.

The MQM recently quit the PPP-led coalitions that rule both Sindh, of which Karachi is the capital, and the federal government. ANP is still a partner.