At least three people died and many others were injured by a bomb planted on a bus in Istanbul, where NATO leaders are scheduled to meet beginning Sunday. Earlier, a much smaller bomb exploded in front of a hotel in the Turkish capital, Ankara, where President Bush is scheduled to stay during an official visit Saturday.

A private Turkish television station showed images of the wounded being rushed to hospitals in Istanbul's crowded Fatih commercial district. Reports from the scene say the bomb exploded on a city bus, just outside a state-run blood donation center.

Just hours earlier, two police officers and a civilian were injured in the blast in Ankara, which occurred roughly 150 meters away from the entrance of the hotel where President Bush will stay. Turkish police said the people were hurt when they approached a package containing the explosives, after receiving an anonymous tip.

U.S. officials preparing for President Bush's first official visit to Turkey were believed to be in the hotel during the incident.

Turkish interior minister, Abdul Kadir Aksu, said marginal terrorist groups seeking attention had likely planted the Ankara bomb. It was a small percussion bomb, a type that makes a lot of noise, but usually causes little damage and few casualties.

President Bush is expected to arrive in Ankara late Saturday, and is set to hold meetings Sunday with Turkish President Ahmet Necdet Sezer and Prime Minister Recep Tayyip Erdogan. Mr. Bush is then scheduled to fly to Istanbul for the two-day NATO summit.

Turkish police say more than 13,000 officers will protect the presidential delegation during its stay in Ankara. Security measures in Istanbul, Turkey's largest city, have also been raised in and around the area where the NATO summit will take place.

Concrete barriers have been built around the area that will be closed to traffic. As an additional precaution, navigation through the Bosporus Strait, which bisects the city, will also be halted during the conference.

Concerns about security have mounted in Turkey since November's deadly suicide bomb attacks in Istanbul against two synagogues, the British consulate, and a British-owned bank. More than 60 people died in the attacks, which were claimed by the al-Qaida terror network.