At least three Pakistani policemen were shot dead and 70 more wounded when supporters of a banned Islamist party opened fire at a rally on Wednesday, the country's interior minister said.

The Tehreek-e-Labbaik Pakistan (TLP) is protesting over the detention of its leader, arrested in April when the group was outlawed by authorities, and is demanding the expulsion of the French ambassador.

The group has been behind major anti-France protests that earlier this year led to the embassy issuing a warning for all French citizens to leave the country.

"They opened fire on police with Kalashnikovs... three policemen were martyred," Sheikh Rasheed Ahmad told a press conference, adding that eight of those injured were in a critical condition.

The Punjab police chief Rao Sardar Ali Khan said in a separate press conference that four officers died on Wednesday.

The TLP in turn accused the police of firing on the crowd, with four of its supporters killed.

Police in Punjab province, of which Lahore is the capital, have denied using rubber bullets or guns and would not comment on claims that protesters had died.

"We have used no such weapons against them," police spokesman Mazhar Hussain told AFP.

Police have said they have used tear gas and batons to control the crowds.

The latest demonstration began on Friday in the group's stronghold city of Lahore, from where thousands of people have begun slowly moving towards the capital Islamabad.

Clashes between the two sides left two police officers dead on the first day of protests, while the TLP reported on Saturday that five of its supporters had died.

Police have closed off major roads and junctions leading to the capital, around 300 kilometres from the current protest site.

Information Minister Fawad Chaudhry said Wednesday that Prime Minister Imran Khan and the security services had agreed to treat the TLP as a militant group.

The government earlier this week announced a breakthrough in talks with the TLP, but the march resumed on Wednesday.