Correspondents say several left-wing militant groups are active in Turkey
Three people, including a policeman and a suspected left-wing militant leader, have been killed in a raid on a flat in Istanbul, Turkish officials have said.
A civilian bystander was also among the dead. Six police officers were injured, some by shrapnel from explosives.
Interior Minister Besir Atalay said the militant was a leader of a little-known group linked to Kurdish separatists.
The raid was one of 60 carried out by police overnight against groups said to be planning "sensational attacks".
More than 40 people were detained in the co-ordinated operations across Turkey's largest city, Mr Atalay said. Weapons, ammunition, bombs and explosives were also seized by the security forces.
The minister said 11 of the suspects were members of the Revolutionary Headquarters, one of whose alleged leaders was killed in the raid on a block of flats in the Bostanci district that began at around 0530 (0230 GMT) on Monday.
Smoke billowed from the block of flats in Istanbul during the raid on Monday
"It is a leftist group which is also linked to the separatist group," he said, referring to the rebel Kurdistan Workers' Party (PKK).
The group had in the past carried out attacks on military targets and an office of the ruling Justice and Development (AK) Party, he added.
The dead militant, who Mr Atalay identified as Orhan Yilmazkaya, had told police by radio that he was from the Revolutionary Headquarters.
"I am a fighter and will fight until the end," he said in one purported conversation broadcast on Turkish television afterwards. "I will probably be killed, but our struggle against fascism, terrorism will go on as it did in the past."
Istanbul Governor Muammer Guler said three police officers had been injured when a grenade thrown by the militant had exploded nearby.
"In a second explosion, four more officers were wounded. They were rushed to nearby hospitals," he told the NTV channel.
One of the officers was initially said to be in a critical condition. Turkish media later reported that a senior commander had been killed.
A civilian standing outside the building died after being shot in the head. Television footage had earlier shown a man lying motionless on the ground nearby in a pool of blood.