Security personnel and people gather near the site of a bomb explosion at Sibi in Balochistan on Tuesday. AFP

Police in the Balochistan provincial capital of Quetta said a bomb planted on a motorbike detonated as supporters rallied for the Pakistan Tehreek-e-Insaf (PTI) party of jailed ex-prime minister Imran Khan.

"The PTI rally was passing by there but it is not clear whether the rally was a target or not," senior police official Farhan Zahid told AFP.

Waseem Baig, a provincial health department spokesman, said six people were wounded in addition to the four killed.

In a statement, PTI said three of its activists were among the dead at the rally -- a convoy of motorbikes and cars parading through the city to curry support for a candidate ahead of national elections scheduled for February 8.

A party spokesman said it was too soon to say whether they had been deliberately targeted.

Pakistan's electoral commission said it was investigating.

The attack came just hours after PTI founder Imran Khan was sentenced to ten years in prison on charges of having leaked classified state documents.

Imran and PTI say they are suffering under an unprecedented crackdown designed to prevent their return to power through the Feb.8 poll.

Agence France-Presse