Six people were shot to death on Tuesday when the security forces opened fire on a demonstration against civilian deaths in the ongoing military offensive in the restive north-west Swat valley, witnesses said.

The demonstration started on Monday after a mortar round hit a house in the Allah Abad area of Swat district in North West Frontier Province, killing one woman and her two children.

Residents transported the victims' bodies about 20 kilometres to Mangora city, where the army headquarters is located. More than 8,000 people joined them in the protest.

The protests gained pace on Tuesday when some 20,000 people violated the curfew and gathered in Mangora, chanting slogans against the military, which is carrying out an operation against the Taliban militants in the area.

"The government forces came here to end the militancy but the militancy has increased instead. Militants are doing well but the civilian population is suffering," a speaker named Mohammad Ali told the rally.

He claimed more than 600,000 people have been displaced and hundreds more dead because of the conflict and the population was facing shortages of food and medicines.

The angry demonstrators set fire to two commercial banks and pelted stones at the law enforcers, which responded with tear gas and gunshots, leaving six people dead and five injured.

Swat, once a popular tourist destination, has seen heavy fighting between government forces and Taliban militants since last October when Islamabad sent thousands of troops to quell the rebellion of the followers of radical cleric Maulana Fazlullah.

Fazlullah launched an armed campaign to enforce Taliban rule in the area. The new government in Islamabad initiated peace talks with militants immediately after it took over in March, but resumed the offensive a month later when the negotiations failed to bring peace to the region.