Four people have been killed and several injured in bomb attacks in Istanbul and the Turkish capital city, Ankara.
The Istanbul attack comes ahead of Monday's Nato forum there
The blasts come ahead of a visit by US President George W Bush for a summit of Nato leaders.
The four deaths occurred in the Istanbul explosion which ripped through a bus on the city's European side.
Earlier, a device exploded near the hotel in Ankara where Mr Bush is due to stay.
Two people were injured.
Turkish police have arrested three suspects, two men and a woman, who were believed to have been on board the bus when the explosion occurred, the state-run Anatolia news agency reported.
A small radical Marxist group, MLFP-FESK, later said they placed a parcel bomb outside the Hilton Hotel in Ankara, private NTV television reported.
The Marxist-Leninist Communist Party had already carried out minor attacks on official targets.
Police declined to comment on the claim, the Associated Press said.
In Istanbul the blast left 15 people injured, according to Korhan Taviloglu, a doctor at the Capi Medical Centre where the victims were being treated.
Istanbul Governor Muammer Guler said the bomb on the bus was being carried by a woman in her early 20s, and that it is believed to have exploded at the wrong time in the wrong place.
"It is understood that the target was neither the bus not the passengers aboard," Mr Guler said.
He added that the bomb was a so-called sound bomb, which would have caused far less damage had it not gone off in a confined space.
A witness Necdet Devrim, told NTV television she saw "injured people on the floor".
"They were screaming and bleeding. Arms and legs were on the street - it was an awful scene."
In a separate incident, Turkish police on Thursday defused a time bomb in the city of Yalova in the north-west.
The home-made bomb, consisting of a mixture of fertilisers and diesel, had been placed outside a car shop in the outskirts of the city.
The White House made it clear that the president's visit would go ahead as planned, despite the attacks.
"As for the schedule, nothing has changed," spokesman Scott McClellan told reporters in Washington, adding that the attacks were an attempt to disrupt preparations for the Nato summit.
Security has been tight in Turkey in the run-up to Monday's summit in Istanbul, which up to 50 world leaders are expected to attend.
Dozens of suspected extremists have been held in police raids over the past few weeks, and about a half-dozen of small bombs have gone off in Istanbul in recent days, injuring several people.
Last month, the authorities claimed to have foiled a plot to bomb the Nato meeting after police arrested nine alleged members of radical Islamic group Ansar al Islam, believed to have links with al-Qaeda.
Fears of terrorist attacks have been running high in Turkey since November, when Istanbul was rocked by a series of deadly explosions.
Sixty-two people were killed and hundreds injured when two synagogues, the British consulate, and a branch of the HSBC bank were targeted by suicide bombers.