KARACHI: A suicide bombing targeting a Shiite Muslim procession in Pakistan's largest city of Karachi killed 25 people and wounded dozens more Monday, as Shiites across the country marked the key holy day of Ashoura.

Violence broke out in the aftermath of the bombing, with shots fired into the air and outraged Shiites hurling stones at security forces who had been guarding the march for their failure to prevent it.

The bombing was the latest in a wave of violence to hit Pakistan since the army started taking on Islamist militants allied with al-Qaida and the Taliban, with terrorist strikes killing 500 people since October.

Karachi has largely been spared the Taliban-linked violence that has struck much of the rest of the country. But the city has been the scene of frequent sectarian, ethnic and political violence.

Extremists from the majority Sunni community regard Shiites as heretical, and the two groups have long engaged in tit-for-tat killings in Pakistan.

After Monday's blast, protesters set fire to a market, two other buildings and several vehicles, smashing shops as others at the procession attempted to stop them. Police and paramilitary troops fired into the air to disperse the crowd.

Television footage showed police cars and ambulances damaged, with windows smashed and doors and hoods ripped open.

Karachi Mayor Mustafa Kamal appealed for calm.

"I want to appeal to the people, to my brothers, my elders to stay calm. I am hearing people are clashing with police and doctors. Please do not do that. That is what terrorists are aiming at. They want to see this city again on fire," he said.

At the nearby Civil Hospital, relatives cried and beat their chests as the wounded lay on stretchers and beds.

"So far, according to information, more than 25 people have been (killed) and more than 50 are wounded," Interior Minister Rehman Malik said.

The minister said the attack was carried out by a suicide bomber who detonated his explosives at the start of the procession.

"These are people who are against the democracy, against our religion, against our Pakistan," he said.

Bomb disposal squad official Munir Sheikh said some 35 pounds (16 kilograms ) of high explosive were used in the bombing.

Security has been tightened across Pakistan for Ashoura, which is the 10th day of the holy month of Muharram, a month of mourning commemorating the seventh-century death of the Prophet Muhammad's grandson. Muharram has often been marred by bombings and fighting between Pakistan's Sunni Muslim majority and its Shiite minority. Some parts of the Muslim world celebrated Ashoura on Sunday.

Malik, the Interior Minister, said he had appealed to the Shiite community to cancel processions for the next two days.

"That does not mean that we are trying to interfere in their religious affairs, but we are doing it for the sake of security and to save precious lives," he said.

Live television footage of the procession showed the blast and smoke billowing from the scene. Ambulances rushed to and from the area.

"I fell down when bomb went off with a big bang. My mind stopped working," said Naseem Raza, a 26-year-old who had been in the procession. "Sometime later, I stood up. People were running. I saw walls stained with blood and splashed with human flesh. I saw bloodstained people lying here and there."

Karachi lawmaker Farooq Sattar condemned the attack as "purely an act of terrorism. It is an attempt to destroy peace of this commercial hub. It is a work of the enemies of Pakistan."

Monday's suicide bombing was the third explosion in as many days to hit Karachi, although authorities attributed a blast which wounded 30 on Sunday to a buildup of gas in a sewage pipe.

Protests broke out after that blast too, with Shiites torching at least three vehicles.

On Saturday, another blast near a Shiite procession wounded 19 people. Authorities attributed that explosion to a firecracker. Abdul Rauf, an official with the bomb disposal squad in Karachi, said the firecracker blast had left a crater in the road.

Monday's attack came after a suicide bomber struck a Shiite procession Sunday in Muzaffarabad, the capital of Pakistan-controlled Kashmir, killing eight people and wounding another 80. That bombing was a rare sectarian attack in an area police say has little history of militant violence. The dead included three police officers, said police official Yasin Baig, adding that another 10 police were among the wounded.

He said it was the first time a suicide bomber attacked a Shiite gathering in the region.

Muslim militants have fought for decades to free Kashmir, which is split between India and Pakistan and claimed by both, from New Delhi's rule. But while Pakistani Kashmir has served as a base for anti-India insurgents to train and launch attacks, the region has largely been spared violence, with militants focusing on the Indian-controlled portion.