A series of arrests have been made in co-ordinated raids across Europe targeting a Turkish extremist group.
The group aims to install a Marxist government in Turkey
Istanbul police arrested 37 suspected members of the Revolutionary People's Liberation Party-Front, DHKP-C, which has carried out many attacks in Turkey.
Another 16 were held in Italy, Germany, Belgium and the Netherlands.
The group, which aims to topple the Turkish government and replace it with a Marxist one, is branded as a terror organisation by the US and the EU.
The main thrust of its political stance is hostility to what it calls American imperialism and Washington's allies in the Middle East - Israel, Britain and Turkey itself, the BBC's diplomatic correspondent Barnaby Mason says.
The group - which has been active since the mid-1970s - has admitted carrying out two suicide bomb attacks in Istanbul in September 2001 that killed three policemen and one Australian citizen and wounded 28 people, mostly police officers.
It has carried out other bombings since 2001.
A leading Turkish counter-terrorism expert told the BBC that recent intelligence suggested that DHKP-C has been trying to regain prominence by imitating the style of al-Qaeda.
"This group (has been) recently inspired by al-Qaeda, although they are not in the same line of argument," Dr Ihsan Bal from the Police Academy in Ankara said.
The early morning raids come after a year of investigation into the activities of the group - begun by Turkish and German police.
Perugia is a possible target, Italian authorities had warned
German and Italian investigators subsequently discovered that the group was also active in the Netherlands and Belgium and co-ordinated the European crackdown outside Turkey, a Turkish interior ministry official said.
"Thirty-seven people were arrested in Istanbul and 16 elsewhere," the official said.
Authorities in Italy arrested three Italian and two Turkish nationals.
The Italian raid centred on the town of Perugia - which has a university for foreigners.
At a news conference, Perugia's chief prosecutor Nicola Miriano said telephone and wire taps during the investigation had revealed the flow of funds, arms and information among supporters of the group in many European countries.
"We didn't have information of an attack on Italy, but there was information pointing to the claiming of attacks in Turkey," Mr Miriano said.
"We are convinced that we have eliminated the Italian cell of the group, although we still have to verify other things," the prosecutor added.
One of the Turkish nationals was said to have headed the DHKP-C cell - which was helped by local Italian anti-imperialist campaigners, Italian authorities say.
Among the Italians arrested is Moreno Pasquinelli, spokesman for the Anti-imperialist Camp - a group opposed to the US-led occupation of Iraq.
They had been gathering funds to finance the Iraqi resistance to the occupation.
"We support the armed struggle in Iraq. Our money is to help them, it doesn't matter to us if they use it to buy weapons, Kalashnikovs, or medicines for people," Mr Pasquinelli told the BBC last year.