LAHORE, Pakistan (Reuters) - Police on Friday fired teargas to disperse thousands of activists from a banned Pakistani Islamist group who rallied to demand the release of its leader and the expulsion of the French ambassador over cartoons depicting the Prophet Mohammed.

       Thousands of Islamists from Tehrik-e-Labaik Pakistan (TLP) in the eastern city of Lahore had begun marching toward the capital Islamabad.

       TLP chief Saad Hussain Rizvi has been in detention since April after the government designated the group as a terrorist movement.

       Authorities have been trying to negotiate with the group, which has rallied its supporters outside its Lahore headquarters for the last two days.

       "Our negotiations have failed," one of its leaders, Muhammad Rizwan, told Reuters.

       Activists across Pakistan had been asked to converge in Islamabad, he said.

       Police fired teargas after the TLP supporters attacked a police checkpoint, police spokesman Arif Rana said. They had clashed with the police at several places in the city, he said.

       "They had assured us that they will remain peaceful but they turned violent," he told Reuters.

       A TLP media coordinator Saddam Bukhari said the police attacked the peaceful rally that was on its way to Islamabad.

       The main arteries to and from Lahore and Islamabad were blocked with shipping containers to prevent demonstrators reaching the city centre. The diplomatic enclave in Islamabad, where most foreign embassies are located, was also blocked off.

       Authorities arrested Rizvi and banned the group in April after its members blocked highways, railways and access routes to cities, abducting and assaulting police and burning public property. Five policemen were killed and three activists then.

       Nearly 1,000 police were injured in the April clashes, which were halted only after government announced a parliamentary vote on expelling the French ambassador over the publication of caricatures of the Prophet Mohammad by the Paris-based satirical magazine Charlie Hebdo.

       The magazine first published the cartoons in 2006, outraging millions of Muslims who considered them deeply insulting. It republished them last year to mark the opening of a trial over a deadly attack on the magazine by Islamist militants in 2015.

       (Writing by Asif Shahzad; Editing by Angus MacSwan)