Several airport workers were injured by a small explosion that occurred while they were cleaning a domestic airliner parked at a gate Tuesday hours before President Bush was scheduled to fly out of the country.

The explosive device was apparently attached to a wallet left in a bathroom on the Turkish Airlines plane, according to a physician who was on the scene. When a cleaner picked up the wallet, it exploded in his hand, said Huseyin Kuytak, a doctor who works for the airport clinic.

"It was a very, very small explosion," he said. "There was not much damage in the plane. But because it's inside a plane, that makes it very significant."

Bush later left Istanbul aboard Air Force One without incident, news agencies reported.

A Turkish Airlines spokesman told the Reuters news agency that three cleaners boarded the Boeing 737-800 at 12:30 p.m. (5:30 a.m. EDT) after passengers had disembarked. It had been due to fly on to Stockholm later. The plane had flown from the Aegean port of Izmir in Turkey.

"One of the workers picked up a wallet from the ground and there was an explosion. He lost a finger and the two others were injured but only slightly," the spokesman told Reuters.

The device contained no electronic components, which might have made it easier to get through screening at Izmir, officials said.

In central Istanbul, hundreds of anti-NATO protesters clashed for a second day with riot police, who fought them with tear gas and pepper spray and beat demonstrators with batons, wire services reported.

Security was extremely tight for the two-day summit, attended by Bush and dozens of other world leaders in Turkey's biggest city.

A series of small explosions in the run-up to the summit, blamed on leftist groups, left nerves on edge in a city that still bears the scars of four al Qaeda-linked suicide bombings last November that killed more than 60 people.

Leftist groups have long been active in Turkey. They claimed a bomb in Istanbul last week that killed four people, including the bomber.

Police said a loud blast near a government building in Ankara Monday night was caused by an exploding security camera rather than a bomb.

Barbash reported from Washington.