"He was arrested with the intention of him being extradited." the police spokesman said. A Russian foreign ministry spokesman declined to comment on the report that Ocalan flew from Moscow.
Mizgin Sen, European spokeswoman for the PKK's political wing, the National Liberation Front of Kurdistan (ERNK), said Ocalan had requested political asylum.
"He is here and he has asked for political asylum ... We are waiting for his application to move forward," she said.
"He decided to go to Italy to apply for political asylum."
Ocalan gave himself up after being stopped at passport control, the police spokesman said. He is now being held at a prison the spokesman said, but declined to disclose its whereabouts. Media reports said Ocalan was being held at Rome's Celio military hospital.
"He is being held in a hospital. He has a slight health problem." the ERNK spokeswoman said. She had no details but said: "It's not too serious. This is what we have been told by Italian officials."
Ocalan, also known as "Apo", has spearheaded an armed campaign for Kurdish autonomy for the last 14 years, in which more than 29,000 people have been killed.
Ocalan, who is believed to be around 50, asked for political asylum in Russia earlier this month. On November 5, one-third of the Greek parliament renewed an invitation to Ocalan to visit Greece, calling him "the leader of the world's most oppressed people."
In October, Turkey threatened Syria with military force if it did not expel Ocalan, who Ankara said had taken refuge in the country.
Turkish authorities said Ocalan used bases in Syrian-controlled territory to launch attacks in southeastern Turkey.
The capture of the rebel leader will be seen in Ankara as a major coup against the PKK.
"Italy should extradite Ocalan back to Turkey." Turkish Interior Minister Hasan Denizkurdu told the Kanal D television channel.
The police spokesman in Rome said he had no details of extradition plans and that the matter was in the hands of Italy's justice ministry.
Ocalan is the main architect of armed Kurdish nationalism. He founded the PKK in 1974 as an extreme left faction that later earned a reputation for ruthlessness. - ReutersReuse content