Correction, March 28: An earlier headline on this story described the protesters as pro-blasphemy. This was wrong. They were demonstrating in support of Mumtaz Qadri, who was executed for the murder of Punjab governor Salman Taseer. The governor had voiced opposition to Pakistan's law on blasphemy.

Thousands of protesters have clashed with police near parliament in the Pakistani capital, Islamabad, against the execution of a man charged with murdering a governor for his criticism of blasphemy laws.

Police fired tear gas to stop an estimated 25,000 protesters, some of whom tried to forcibly enter the security zone of the capital, dubbed the red zone.

Al Jazeera's Kamal Hyder, reporting from Islamabad, said that the military was called in to take over security in the red zone.

"Their main task will be to secure the zone that includes parliament, Pakistan Television, the Foreign Office, the Supreme Court and diplomatic enclave," he said.

Protesters marched from the garrison town of Rawalpindi to Islamabad in solidarity with Mumtaz Qadri, who was charged and executed for the murder of Punjab province governor Salman Taseer.

READ MORE: Pakistan hangs Mumtaz Qadri for murder of Salman Taseer

"The crowd is massive and the police used blockades and tear gas on protesters as they were getting out of control," Sami Yousafzai, who was present at the protest site told Al Jazeera.

"They want the blasphemy law to be active and anyone who says anything blasphemous about Prophet Muhammad should be executed."

Peace and tolerance

Taseer was shot 28 times by Qadri, his bodyguard, in January 2011 for defending a Christian woman jailed on blasphemy charges.

The protest leaders said they would hold a sit-in until their demands were met, but did not specify what their demands were.

If our people don't understand the Sharia properly, our community will never be educated. They will always go for violence instead of peace and tolerance. Dr Mufti Akeel Ur Rehman Pirzada, Chairman of the Pakistan's Ulama Aman committee

According to Dr Mufti Akeel Ur Rehman Pirzada, chairman of the Pakistan's Ulama Aman committee - a body of Islamic scholars to promote peace - the protest is aimed to press the government for certain demands and "cause unrest".

READ MORE: Hanging revives Pakistan capital punishment debate

"Their first demand is to punish people convicted of blasphemy, for example Asia Bibi who was charged with allegedly desecrating the Quran," he told Al Jazeera.

"If our people don't understand the Sharia [Islamic law] properly, our community will never be educated. They will always go for violence instead of peace and tolerance."

About 100,000 people attended Mumtaz Qadri's funeral and he has been labelled as a hero, highlighting the support for such people in the society.

The protest was blacked out by the Pakistani media after the government banned it fearing unrest.

Singer-turned preacher attacked

Meanwhile, Junaid Jamshed, a pop star-turned preacher, was attacked when he was leaving Islamabad airport on Saturday night.

He has been mostly living abroad since 2014 when he made a comment that was said to be an insult to one of the wives of the prophet.

After the attack, Jamshed tweetted: "I wish I wasn't travelling alone. A little distressed with what happened."

"Its about time [we] as a nation decide that [we] will not let these religious fanatics prevail amongst us."

More than 100 people are charged with blasphemy each year in Pakistan, a predominantly Muslim nation of 180 million, many of them Christians or people from other minorities.

Conviction of blasphemy carries a death sentence in Pakistan, but no one has yet been hanged.