Turkish police fired tear gas at more than 150 left-wing demonstrators who hurled rocks and tried to smash through a barricade in Ankara on Saturday, just hours before President Bush (search) arrived for a NATO (search) summit.
Police used bomb-detecting dogs to comb the area around the hotel where Bush is expected to stay, and security officials closed streets throughout the capital, part of a massive operation following Thursday bombings in Ankara (search) and Istanbul that killed four people and injured 17.
Also Saturday, supporters of Jordanian-born militant Abu Musab al-Zarqawi (search) said they had kidnapped three Turkish workers in Iraq, Arab TV station al-Jazeera reported.
An al-Jazeera employee told The Associated Press that the group threatened to behead the hostages within 72 hours if Turkish companies did not withdraw from Iraq.
Bush is unpopular in Turkey, where the overwhelming majority of the public opposed the war in neighboring Iraq. If Turkish hostages were executed in Iraq, that could considerably increase tensions.
In the Turkish capital of Ankara, about 6,000 people, mostly members of trade unions and leftist groups, gathered before Bush's arrival, with some chanting, "Murderer U.S.A., get out of the Middle East!"
About 150 people broke away from the rally and marched to a police barricade, chanting, "We will go beyond barricades protecting Bush!" and "We will make Bush's Ankara visit hell!"
Police then fired tear gas at the group from an armored personnel carrier.
A few minutes later, the group, called the Socialist Platform of the Downtrodden, again attacked the barricade, throwing rocks at police. Officers again responded with tear gas.
Organizers then asked everyone to disperse.
Police said 13 officers were injured in the clash, the Turkish news agency Anatolia reported early Sunday.
On Sunday morning, armored police vehicles took up positions between the prime minister's residence and the presidential palace — where Bush is scheduled to meet with Turkish leaders. Hundreds of police officers lined the streets.
Bush's visit was preceded by a series of protests and bomb blasts, including one Thursday that injured three people outside the Ankara hotel where Bush was expected to stay. Another blast that day on an Istanbul bus killed four people and injured 14 others.
On Saturday, a small bomb attached to a banner protesting the summit and Bush's visit went off in downtown Istanbul, causing no injuries.
Two small bomb blasts overnight caused minor damage but no injuries in the southern city of Adana, and police defused a remote-controlled bomb placed under a car in the Black Sea port of Zonguldak, the Anatolia news agency said.
The bombings have been blamed on militant leftists, and Turkish police have detained scores of suspected members of radical groups.
Militant Kurdish, Islamic and leftist groups are active in the country, and security in Istanbul has been of special concern since November, when four homicide truck bombings blamed on Al Qaeda killed more than 60 people.
More than 23,000 police officers will be on duty during the NATO summit, which also will be attended by British Prime Minister Tony Blair and French President Jacques Chirac, among other leaders.
Bush meets with Turkish leaders early Sunday before heading to Istanbul for the NATO summit.
Turkish commandos patrolled the Bosporus Strait bisecting Istanbul in rubber boats with mounted machine guns, while bomb squads used trained dogs to comb the streets around the Ankara hotel where Bush was expected to stay. Roads throughout the two cities have been closed.
"All necessary measures have been taken for the NATO summit," Deputy Prime Minister Abdullatif Sener said. "People with bad intentions will not be given a chance."
NATO Secretary-General Jaap de Hoop Scheffer visited Istanbul Mayor Kadir Topbas to apologize for the inconveniences.
Scheffer handed flowers to the mayor.
"The people of Istanbul are uncomfortable because of the NATO summit," Scheffer said, handing the mayor flowers. "We brought these flowers to apologize."