The Turkish police in the city of Bursa on Monday claimed the suspects were members of the Islamist group Ansar al-Islam, believed to be linked to al-Qaida.
The statement making the claim gave no further details, but media reports said the governor of Bursa was expected to make a detailed statement later.
Turkey is under pressure to ensure water-tight security at the North Atlantic Treaty Organisation (NATO) summit, which is scheduled for 27-28 June and will be attended by dozens of world leaders, including US President George Bush.
Fears over "terrorist" attacks in Istanbul, Turkey's largest city which straddles the Bosphorus, have been heightened since four bombings targeted two synagogues, the British consulate and the HSBC bank in the city last November. Sixty two people were killed and hundreds injured.
Officials have blamed the November bombings on local Islamic fighters with links to al-Qaida and have indicted 69 people over the attacks.
Ansar-Al Islam, founded in December 2001 by the Norway-based Iraqi Kurd Mullah Krekar, used to control a small enclave in northeastern Iraq before it was reportedly crushed by invading US forces in March last year. It then had about 700 to 900 members.
On 22 March, the United States added Ansar al-Islam to its official list of "terrorist" organisations, saying it was linked to al-Qaida and had mounted attacks against US-led occupation forces in Iraq.