Supporters of Tehrik-e-Labiak Pakistan celebrate after capturing a police vehicle during their protest march toward Islamabad on Wednesday. AP

At least three more Pakistani policemen were shot dead and 70 more wounded when supporters of the banned Tehrik-e-Labbaik Pakistan (TLP) party opened fire at a rally on Wednesday, the country's Interior Minister Sheikh Rasheed said.

The outlawed TL) 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.

The group is protesting over the detention of its leader Saad Rizvi, arrested in April when the TLP was outlawed, and demanding the expulsion of the French ambassador.

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

TLP workers celebrate after capturing a police vehicle during their protest march toward Islamabad. AP

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 said.

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 in Lahore on Friday left two police officers dead, while the TLP reported on Saturday that five of its supporters had died.

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

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

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

"We tried our best for the success of the talks but the government is not serious about fulfilling its commitments," TLP spokesman Sajjad Saifi said.

"The expulsion of the French ambassador is our main demand."

Four policemen killed in Lakki Marwat ambush

Unidentified gunmen attacked a police patrol overnight in northwest Pakistan, killing four before fleeing the scene, a police official said on Wednesday.

No group claimed responsibility for the attack in Lakki Marwat, a town in the Khyber Pakhtunkhwa province bordering Afghanistan. Police official Umar Khan said a search operation for the culprits was still underway.

Khan provided no further details and only said the funeral of slain officers was held on Wednesday morning.