At least four people have been killed ahead of an election rally that Pakistan's former Prime Minister Nawaz Sharif was preparing to attend.
Mr Sharif has been campaigning around the country
Members of Mr Sharif's PML-N party say they came under attack from supporters of the rival PML-Q party which backs President Pervez Musharraf.
The shooting took place near Pakistan's main garrison town, Rawalpindi.
Correspondents say it is the worst election-related violence ahead of polls due on 8 January.
The elections are for national and provincial assemblies. They come weeks after President Pervez Musharraf resigned as head of the army and was sworn in for another term as president, this time as a civilian.
Some reports say that supporters of Mr Sharif were trying to hang election banners close to the houses of PML-Q supporters when they came under fire.
Mr Sharif was some about two kilometres away when the shooting started, a spokesman for his party, Sadiq ul-Farooq said, the Associated Press news agency reports.
"Nawaz Sharif and his procession are safe," Mr ul-Farooq said.
The PML-Q party was formed by defectors from Mr Sharif's PML party after he was deposed by the then General Musharraf in a coup in 1999.
The main power contest between the PML-Q and PML-N is in Punjab province, which accounts for more than half of the country's total votes.
Another former Prime Minister, Benazir Bhutto, is also due to address an election rally in Rawalpindi on Thursday.