KARACHI, PAKISTAN, JULY 14 -- At least 72 persons died and more than 250 were injured when two car bombs exploded in a busy shopping area in downtown Karachi tonight, according to police sources and doctors at the city's two main hospitals.

Officials described the blasts as the bloodiest terrorist incident ever in Pakistan, where bomb attacks have become more frequent in recent years.

Soon after the explosions, which rocked the heart of the city, crowds of youths took to the streets in many Karachi districts to chant slogans against 3 million Afghan refugees who have fled to Pakistan in the almost eight years since the Soviets invaded Afghanistan.

Heavy police patrols were ordered for areas frequented by the refugees.

No group claimed responsibility for today's attack. City authorities issued a statement blaming "saboteurs of foreign origin" for the explosions but did not elaborate.

Pakistan supports rebels fighting the Soviet-backed communist government in Kabul, the Afghan capital, and one senior Pakistani security official attributed tonight's bombings to the KGB-backed Afghan secret service.

There have been numerous terrorist bombings in Pakistan in the past two years, but tonight's was the first in Karachi, Pakistan's largest city and home to about 35,000 Afghan refugees.

Three explosions in Lahore on July 5 killed seven persons and injured more than 50. A bomb blast last night near the U.S. Consulate in Peshawar, near the Afghan border, caused no injuries.

Some observers have speculated that some earlier bombings were the work of Pakistani dissidents who oppose the government of Prime Minister Mohammad Khan Junejo, which has the support of President Mohammad Zia ul-Haq.

Last week, Iranians loyal to Ayatollah Ruhollah Khomeini attacked dissident countrymen living in exile in Karachi and the western Pakistani city of Quetta.

There have been no arrests in connection with the recent spate of bombings.

Witnesses tonight said that bombs planted in two cars parked near the main bus terminal and in a crowded bazaar nearby exploded within about 20 minutes.

The first explosion destroyed eight shops and a hotel on the ground floor of a four-story building, killing several roadside shopkeepers.

Reporters saw volunteers recovering body parts from a rooftop at the site.

As thousands fled for safety, the second explosion rocked the area.

The blast touched off a fire, engulfing a three-story building full of shoppers, workers and residents.

Volunteers walked in pools of blood inside several shops to recover the dead and injured. The city's entire fleet of 200 ambulances was called out.

An emergency was declared in Karachi's hospitals, which were hampered by a severe shortage of medical supplies.

Pakistani intelligence officials said tonight's bombings were similar to the earlier explosions elsewhere in the country, which occurred at peak hours in crowded urban centers.

The bombs are "aimed at generating hatred against Afghan refugees and a sense of insecurity among the masses," said a security official.

Victims' frenzied relatives, who had gathered outside the two hospitals tonight, shouted slogans against Afghan and Iranian refugees.