Shiite Muslims enraged by a mosque bombing that killed 20 worshippers battled police and burned American fast-food restaurants Tuesday as the government struggled to contain a third day of violence in Pakistan's largest city.

Mass funerals for the victims of Monday's attack sparked what appeared to be orchestrated rioting as hundreds of youths rampaged near the wrecked Imam Bargah Ali Raza mosque (search), stoning police and setting fire to shops and buses.

Along a quarter-mile battle-scarred stretch of Karachi's main road, men with guns took up four or five positions on rooftops and fired at police and paramilitary rangers.

Police Chief Asad Ashraf Malik said four policemen were wounded by gunfire, and that 150 had been arrested — in addition to more than 50 detained during street riots Monday night.

Asad threatened more force.

"We have decided not to be lenient. If anyone goes on the streets to cause trouble, they will be dealt with strictly," he said.

Karachi, a volatile city of 14 million, is no stranger to armed violence motivated by crime, politics and religion. Sectarian strife between militant elements of majority Sunni Muslims and minority Shiites has only deepened since President Gen. Pervez Musharraf (search) gave his support to the U.S.-led war on terrorism in late 2001.

But the city has endured three unusually turbulent weeks since a May 7 suicide attack at a Shiite mosque killed 22 people. It was followed by bloody clashes during elections that left at least 10 dead and a twin car bombing near the U.S. consul-general's residence last week that killed a policeman and injured 40 others.

The drive-by shooting Sunday of prominent Sunni cleric, Nazamuddin Shamzai (search), made matters much worse, triggering unrest and raising fears of sectarian clashes — a fear magnified after Monday's bombing at the Shiite mosque that killed 20 people and injured 75. Police suspect it was a suicide attack but have few clues about who was behind it.

No one has claimed responsibility for any of the recent attacks.

Musharraf pledged action to stem the bloodshed, but no new measures were announced. Separately, he warned in a speech to representatives of Muslim countries that the world risked plunging into an "abyss of barbarism" unless it tackled the poverty and alienation that feeds Islamic extremism.

Information Minister Sheikh Rashid Ahmed said the president was expected to make "important decisions" in the coming days — perhaps replacing the leadership of the Karachi or provincial security establishment — but appeared to rule out the possibility of emergency rule by the army.

"There could be changes at important places," Ahmed told The Associated Press.

Among the estimated 10,000 mourners who gathered for the funerals of the victims of Monday's bombing, many vented anger at Musharraf, chanting "Death to America" and slogans against his government as they beat their chests in grief.

A few hundred of the mourners started stoning police, who fired tear gas. Rioters set fire to buses, shops and offices near the mosque, leading to exchanges of gunfire. Riots broke out in four or five other places in the city.

Malik, the police chief, said that in all, about 35 vehicles and 12 to 15 shops and restaurants were burned, including a KFC and a McDonald's.

Shiite clerics had earlier urged mourners to show restraint during the mass funerals that proceeded to two different Shiite graveyards, the route patrolled by police in riot gear.

"Everybody who is a Muslim should understand that some elements want to create unrest," said Yousaf Hussain, a Shiite leader. "They are the enemy of Pakistan and Islam, and I ask you to understand this conspiracy and show patience."

The May 26 twin car-bombing near the U.S. consul-general's residence was labeled a retaliation by outlawed terror group Harkat-u-Mujahedden-al-Almi over the recent arrests of seven members.

Harkat is accused of a failed attempt to assassinate Musharraf with a car bomb in 2002 and of a car bombing at the U.S. Consulate in the same year that killed 14 people.