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The people of Pakistan's commercial hub Karachi have voted in a by-election after suspected political shootings overnight killed at least 28 people and wounded another 40 in the southern city.

Mohammad Saghir, the provincial health minister, said armed men attacked people to "ruin the peace of this city" before the polls opened on Sunday.

Zulfiqar Mirza, home minister of southern Sindh province, of which Karachi is the capital, said 60 people had been arrested in connection with the attacks.

Police said they were still investigating the motives behind the shootings, but past targeted killings in Karachi have been linked to gangs controlled by the city's various political groups.

The by-election was called to fill a position left vacant after Raza Haider, a politician in Karachi's dominant Muttahida Qaumi Movement (MQM), was killed two months ago, sparking violence which killed 100 people in a week.

Opposition boycott

MQM's main rival, the ethnic Pashtun-based Awami National Party (ANP), boycotted the poll after complaining that the MQM would rig the ballot.

"Soon after announcing its boycott of the by-election, ANP's terrorists began killing innocent citizens in a bid to sabotage the election process," MQM said in a statement.

The ANP rejected the accusations that it carried out Saturday's killings, the victims of which included MQM workers.

"We are not involved in killings, and I think that this blame game should be stopped," Amin Khattak, a senior ANP member, said.

MQM's leadership said it was weighing all options in response to the violence, including withdrawing from the provincial coalition government led by the Pakistan People's Party (PPP) of Asif Ali Zardari, the president, a source said. It has made similar threats in the past.

The MQM largely represents the ethnic Mohajirs, descendants of Urdu-speakers who migrated from India after the creation of Pakistan in 1947.

History of violence

Karachi often shuts down for brief periods after political killings, as well major bombings.

The city has a long history of sectarian violence and was a main target of al Qaeda-linked fighters after the September 11, 2001, attacks on the United States, when Pakistan joined the US-led campaign against such groups.

Stock market investors keep a wary eye on tension in Karachi, home to Pakistan's main port, stock exchange and central bank - and the main gateway for Western military supplies bound for neighbouring land-locked Afghanistan.

Aside from trying to contain violence in Karachi, the Pakistani government faces a Taliban insurgency and the task of rebuilding areas devastated by summer floods which inflicted $9.7bn in damage and will strain the weak economy for several years.

But hundreds of targeted killings this year have raised concerns that violence would escalate and create a new crisis for the US-backed federal government.

Source: Al Jazeera and agencies