A Pakistani politician has been shot dead at a mosque in the southern city of Karachi, sparking deadly protests.
Raza Haider, of the MQM political party, was gunned down with his guard as he attended a funeral near the centre of Pakistan's business capital.
The killing triggered violence in which at least 34 people were killed and more than 100 wounded, officials say.
More than 300 people have been killed in political killings in Karachi this year, say human rights organisations.
Witnesses say that four gunmen escaped after opening fire on the member of the Sindh provincial assembly as he performed ablutions on Monday.
As news of the assassination spread, dozens of vehicles were set alight and armed demonstrators blocked major roads in Karachi.
Violence was also reported in other parts of Sindh province, including Hyderabad and Sukkur.
Markets and shopping centres have been closed, and traffic jams built up as people fled home to safety.
Witnesses said armed men were moving freely around several Karachi neighborhoods - including Martin Quarters and Nazimabad - targeting civilians.
Police surgeon Hamid Parehyar told the BBC that they had received 27 bodies of people killed in the violence, and another four deaths have also been reported.
Pakistani Prime Minister Yusuf Raza Gilani appealed for calm and said he had ordered an inquiry into the killing.
Leaders of the MQM (Muttahida Qaumi Movement) claimed supporters of the rival Awami National Party (ANP) - which is dominated by the Pashtun community - were behind the assassination.
Police officials said almost all of those killed or injured belonged to the Pashtun community.
Both parties are part of Pakistan's governing coalition, which is led by President Asif Ali Zardari's Pakistan People's Party.
"Every MQM activist will now be looking to pay back the people involved in this heinous act," said Babar Ghauri, a senior MQM leader and the federal government's ports and shipping minister.
Another senior MQM leader, Faisal Subzwari, told the BBC Mr Haider had been receiving death threats.
Mr Haider belong to the Shia community. Dozens of Shia have been killed in Karachi since a Shia religious procession was bombed on 30 December 2009.
However, Mr Subzwari said the recent threats had all come from the ANP.
The ANP vehemently denied any involvement.
"The MQM is speaking out in grief. We demand that a through investigation be conducted to find the culprits," Shahi Syed, president of the ANP in Sindh, told the BBC.
Mr Haider's assassination is the most recent in a string of political and sectarian killings that have struck Karachi since late 2009.
Security officials in Karachi have told the BBC that, despite the MQM's accusations, there is evidence to suggest the assassination was carried out by the Lashkar-e-Jhangvi, Pakistan's deadliest militant group.
The group has links to al-Qaeda and many of its leaders are based in Pakistan's tribal region.
The MQM has declared three days of mourning, starting with Mr Haider's funeral on Tuesday.
Schools and well as businesses - including the Karachi Stock Exchange - are expected to remain closed during this period.