Small bombs exploded in Turkey's two largest cities Thursday in advance of a summit of leaders of NATO alliance countries that starts in Istanbul on Monday.

One bomb was left in a package a few dozen feet from the entrance of the Ankara hotel where President Bush is scheduled to spend Saturday night. The bomb mangled the foot of the Turkish police officer who tested the bag's contents with a gentle kick. He was given first aid by U.S. security officials who rushed from the hotel, where they were overseeing preparations for Bush's arrival.

The second blast killed four people and injured 14 aboard a municipal bus in Istanbul. The fatalities included the bomber, a 20-year-old woman who was a member of a Marxist-Leninist militant organization, according to Muammer Guler, the governor of Istanbul. He did not name the group.

Guler said the bomb apparently went off prematurely, while being carried in the woman's lap to an unknown destination. "It was not a suicide bomb," he said outside the hospital where the wounded were carried, half a block from the tree-lined commercial street where the explosion occurred at about 3:50 p.m.

The bomb went off as the accordion-style bus was moving on a busy street through a largely residential neighborhood, miles from the NATO summit site or any other obvious target in the city of 12 million.

"What we saw afterward, the people carrying the injured, a lot of people running, it was no different than what you see on TV from Israel," said Murat Sertkaya, 27, who saw the explosion from a nearby drug store.

The bomb was a percussion charge, he said, which typically produces a powerful blast wave and noise but no shrapnel. This one blew the side off the bus about three rows behind the driver.

Responsibility for the Ankara bomb was taken by a group calling itself the Armed Forces of the Poor and the Oppressed, known by the Turkish initials FESK, according to Turkish news agencies. The group is not well known but was described here as affiliated with the Marxist-Leninist Communist Party.

FESK also claimed responsibility for four small bombs that exploded outside branches of a British bank hours before Prime Minister Tony Blair arrived in Turkey in May.

Following Thursday's attacks, the White House press secretary, Scott McClellan, said it did "not appear that these terrorist attacks are an attempt to disrupt the preparations for the NATO summit, which is a gathering of free nations united in our global fight against terrorism. As for the schedule, nothing has changed."

Bush is due to arrive in Ankara late Saturday. After spending the night at the Hilton, a high-rise in the busy diplomatic section of the sprawling capital city, he is scheduled to meet with Turkish leaders on Sunday. He is then scheduled to fly to the country's commercial center, Istanbul, late Sunday for the summit meeting of the North Atlantic Treaty Organization on Monday and Tuesday.

The Ankara explosion occurred near the hotel's main entrance, at the foot of steps leading to a small park. Police began a search of apartment buildings that face the hotel across the tree-lined block.

"I thought it was thunder, because it was about to rain," Mithat Aksoy, a driver waiting in the taxi queue at the hotel entrance, told the television news service CNN Turk. "Then I saw smoke on the left.

"There was a foreigner standing there already, helping the policeman. . . . The policeman's feet were torn up, so we tried to stop the blood and we called the ambulance. He was conscious. "

Several fringe radical groups remain active in Turkey as vestiges of a strong leftist movement that staged attacks in the 1970s. "There are a bunch of these incidents that aren't terribly big but have caused some deaths," said a U.S. official in Turkey, who spoke on condition of anonymity.

Al Qaeda surfaced in Turkey in November, taking responsibility for four car bombs in Istanbul that killed more than 60 people. The attacks caught Turkish authorities by surprise and focused security for the long-planned summit, which more than 50 heads of state and prime ministers plan to attend, toward preventing any spectacular attacks.

Tanker vessels carrying combustible cargos will be barred from the Bosporus Strait that divides the European and Asian shores of Istanbul. AWACS radar planes will circle the skies overhead. A large section of the city will be closed off to all but residents.

Turkish authorities last month arrested several Turks said to be plotting an attack on the summit from Bursa, a city about 50 miles south of Istanbul. More recently, in a possibly separate case whose details remain murky, two men were arrested while in possession of fuses.

The trial of about 60 people charged in the November attacks is scheduled to resume on Monday, the day the summit convenes.

Medics help the injured after a bomb exploded on a bus in Istanbul. Four people died, including the bomber; 14 were hurt. Another bomb injured a police officer near the Hilton Hotel in Ankara, where President Bush is scheduled to stay.