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.

Thousands of supporters of the Tehreek-e-Labbaik Pakistan (TLP) party have begun slowly moving towards the capital Islamabad
Thousands of supporters of the Tehreek-e-Labbaik Pakistan (TLP) party have begun slowly moving towards the capital Islamabad AFP / Arif ALI

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.

Clashes in Lahore left two police officers dead on Friday
Clashes in Lahore left two police officers dead on Friday AFP / Arif ALI

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.

"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."

The government has repeatedly said it is not willing to close the French embassy or expel the current acting ambassador.

The group's leader Saad Rizvi was arrested in April when violent anti-France protests erupted, leading to the deaths of six police officers.

The TLP has waged an anti-France campaign since President Emmanuel Macron defended the right of a satirical magazine to republish cartoons depicting the Prophet Mohammed -- an act deemed blasphemous by many Muslims.

Few issues are as galvanising in Pakistan as blasphemy, and even the slightest suggestion of an insult to Islam can supercharge protests, incite lynchings, and unite most of the country's warring political parties.