The fourth explosion in six days rocked central Baghdad yesterday, increasing speculation that Iran may have acquired surface-to-surface missiles capable of scoring accurate hits on the Iraqi capital from Iranian territory more than 100 miles away.

Iraqi security police quickly sealed off the site after the blast. The dawn sky lit up with a pink flash seconds before the powerful blast shook buildings in Baghdad. Diplomats and witnesses in the Iraqi capital told news agencies there that the explosion occurred near the main office of the ruling Baath Arab Socialist Party, killing 11 people and damaging several houses.

Fighting in the southern Hawizah marshes of Iraq appeared to have subsided yesterday amid conflicting accounts from Tehran and Baghdad over which side had emerged victorious after a week of some of the fiercest battles in the 4 1/2-year-old Persian Gulf war.

As the war continued to be brought home to residents of Baghdad, hundreds of Iranians and foreigners fled Tehran on the last flights out before an 8 p.m. (noon EST) deadline set by Iraq, which threatened to shoot down commercial airliners flying over Iran after that time. Story on Page A16.

Civilians landing in Vienna on an Austrian Airlines plane from Tehran told Reuter the aircraft had taken off from the Iranian capital amid antiaircraft fire and screaming sirens. Exhausted and disheveled passengers said Tehran airport was a shambles, packed with hundreds of people clamoring to leave.

Austrian Airlines manager in Iran, Kurt Reimann, said he saw bombs falling close to the airport before his flight took off.

A French woman married to an Iranian, who arrived in Paris on the last foreign airliner to depart Tehran before the deadline, told reporters there, "There are lots of civilians being hurt in the bombing. The hospital where my husband works is overflowing with wounded."

An Iraqi military spokesman said warplanes raided "selected targets" in Bushehr, Hamadan, Erdebil, Karand and Khorramabad to "force the rulers of Iran to accept peace and end the war."

Tehran radio said eight Iranian civilians were wounded and four houses were destroyed in the raid on Hamadan, 240 miles southwest of Tehran. The broadcast also said Iraqi jets had fired four rockets into the coastal city of Bushehr, "without causing any casualties."

Iran's Prime Minister Hossein Mousavi called Iraq's threat against commercial planes flying over Iran "insane" and said Iranian missiles would destroy Baghdad's airport if the threat is carried out.

Yesterday's blast in central Baghdad came only hours after Egyptian President Hosni Mubarak and King Hussein of Jordan left the Iraqi capital after a surprise one-day visit that was viewed as an effort to lend moral support to Iraq's President Saddam Hussein.

There were suggestions in diplomatic circles in Washington and Tehran that Iran recently had acquired missiles, probably Soviet-made, from unknown sellers. The speculation here was that Libya and Syria, Soviet allies and arms purchasers, may have provided the weapons. The Soviet Union, Iraq's main supplier, is said to be angry about the reports and to have told European diplomats that it is not providing weapons to Iran.

After each of the first two explosions in Baghdad last week, Iraqi military spokesmen dismissed Iran's claim that they had been caused by surface-to-surface missiles from inside Iran and U.S. analysts tended to believe them. But Iraq's silence after the last two has fueled speculation that the claims may be true.

Agence France-Presse, quoting unnamed sources in Tehran, said Iranian forces were reinforcing positions east of the Tigris River after a severe setback Monday in their offensive along the strategic Baghdad-Basra highway north of the town of Al Qurnah.

Military experts in Tehran said if Iran can consoldiate positions in the Hawizah marshes, which cover more than 800 square miles, and set up antiaircraft defense, Iraqi troops will find it very difficult to regain lost ground.

Foreign and Iraqi journalists escorted over a portion of the marshes yesterday reported little sign of continued fighting and indications that Iran had sustained substantial losses. Iraqi military commanders said the fighting in the marshes had been tough but said they had pushed Iranian forces back to positions they held before this latest offensive. They acknowledged that Iran still held the oil-rich Majnoon Islands.

Tehran radio said the first batch of Iraqi prisoners from the battles in the marshes would be arriving by railroad in the Iranian capital this morning. In an unusual move, they invited foreign journalists to be there for the occasion.