Police detained four suspects and patrolled the high-walled perimeter of the US consulate in Istanbul, after a terror attack which in which three policemen and three gunmen were killed.
Busloads of police were stationed outside the mission, CNN Turk said, and security was tightened at other diplomatic installation. Police manned checkpoints at various point in the sprawling city to check the identity of passing motorists.
Turkish television flashed news of Interior Minister Besir Atalay reporting the detention of four suspects, three of them in Istanbul. There were no immediate details.
The US consulate had already been moved to a more secure location in Istanbul following the September 11, 2001 attacks by al Qaeda terrorists in New York.
Turkish police were investigating media reports that al Qaeda was behind yesterday's attacks, and that the gunmen who carried it out had received weapons training in Afghanistan.
Police were still looking for the fourth man in the squad, who escaped from the scene in a car during the gunbattle between the gunmen and police.
The attack coincides with political tension in Turkey. The governing party is fighting to avoid being banned for alleged anti-secular activities and police are probing a shadowy far-right group suspected of plotting a military coup.
Al Qaeda link in doubt
At a memorial ceremony at police headquarters in Istanbul, lines of police and officials said prayers in front of the three officers' coffins, draped in Turkish flags.
"We saw once more that terror is ruthless, has no religion, faith or nationality. We condemn terrorism from wherever it comes. We will continue the fight against terrorism to the end," the Interior Minister said.
But there was scepticism among security experts that al Qaeda was behind the assault on the consulate, given the small scale and amateurish nature of the attack.
Turkey has seen armed attacks from a variety of groups over the years, including far-left militants, Kurdish separatists and Islamist militants.
The most serious were in November, 2003, when 62 people were killed by Islamists who struck at two synagogues, a bank and the British consulate.
In Turkey's eastern Igdir province, friends and relatives of one of the dead gunmen, named as Bulent Cinar, expressed shock at his involvement, the state-run Anatolian news agency said.
"Bulent was a good boy. We were shocked when we heard what happened. I can't understand how he could do such a thing. He was definitely deceived," said Erhan Karaboga, a friend of his.