KARACHI (AFP) - A Pakistani grocer standing in next month's historic polls was shot dead on Thursday in a drive-by killing in the southern city of Hyderabad, the first candidate to die in the election campaign.

Mr Fakhrul Islam, 46, was a candidate for the secular Muttahida Qaumi Movement (MQM), a coalition partner in the outgoing government which has been threatened by the Taleban.

He was killed by gunmen on motorcycles when he left the shop he owned with his father, police said.

Pakistan's interim prime minister Mir Hazar Khan Khoso ordered an immediate tightening of security for all candidates.

"He sustained four bullets in his head and abdomen and died on the spot," police official Akhtar Hussain told AFP.

Mr Islam's father was not injured, but police said he was in "deep shock".

There was no immediate claim of responsibility. Local officials told AFP that the killing had the signature of Islamist militants.

Pakistan's umbrella Taleban faction has directly threatened the main secular coalition partners in the outgoing government.

The killing of Islam, who was running for the Sindh provincial assembly, is likely to fuel concerns that violence will mar the national and regional elections on May 11.

The polls will mark the first democratic transition in the nuclear-armed country, which has been subject to extended periods of military rule.

The MQM said Islam had been targeted deliberately and accused "terrorists" of trying to sabotage a peaceful democratic process.

"The way he has been killed and his father remained unhurt shows the precision the killers have and also their intention to target him in particular," said party spokesman Wasay Jalil.

"Terrorists are threatening to sabotage elections... but these terror acts will not deter us from taking part in elections and our stance against extremism and terrorism," he added.

Hyderabad is the second largest city in Sindh after the port city of Karachi, which is deeply troubled by ethnic and political killings.