Activists of a radical Islamist group attacked a Shia procession in Pakistan's Punjab province, leaving at least 13 people injured, police said on Sunday.

According to an FIR, the Shia procession was going to Imambargah (congregation hall) in Sialkot, about 130kms from Lahore, in connection with the Chehlum of Imam Hussain on Saturday when a group of people armed with pistols, clubs and stones attacked the mourners.

The Chehlum is a Shia religious observance that commemorates the martyrdom of Imam Hussain, the grandson of Prophet Muhammad, who was martyred on the 10th day of the month of Muharram.

"Over a dozen mourners were injured and were shifted to hospital for treatment," according to the FIR.

A senior police officer told PTI on Sunday that tensions had been simmering over the past few days in the area between the Tehreek-i-Labbaik Pakistan (TLP) and Shia activists over the route of the procession.

"The TLP local leaders wanted that the Shia procession should not pass in front of their mosque cum seminary. The local Shias were determined to go to Imambargah by the same route they take every year," the officer said on condition of anonymity.

"Police officers had held talks with both sects and apparently the TLP leaders agreed that they would not object to the procession route," the officer said.

However, a group of TLP activists came out of their seminary on the Alam Chowk of Shahabpura (Sialkot) and attacked the Shia mourners.

"Thirteen people, all of them belonging to the Shia community suffered injuries. The condition of a couple of them is stated to be critical as they had head injuries," the officer said.

Sialkot Police chief Faisal Kamran said the police force reached the spot after learning about the incident. However, the attackers managed to flee.

The police chief said 30 suspects have been booked under the terrorism charges with no arrest so far.

"Police force has been deployed in the area to control law and order. Besides police teams have been constituted for the arrest of the suspects," he said.

Video clips, shared on social media, showed a good number of people holding clubs and weapons opening fire and attacking the Shia procession participants.

"#ShameOnTLP and #SialkotJuloosAttack" were the top trends on Twitter on Sunday, with most people condemning the attack and demanding strict action against those involved in the incident and also banning the TLP.