ROME (Reuters) - Italian police on Wednesday arrested four men accused of being the leaders of a human trafficking ring which smuggled people from Africa to Europe and forced women into the sex trade upon arrival in Italy.

Another two men were arrested on suspicion of financially supporting the network and selling illegal drugs. The group, which was headquartered in Padua in northern Italy, had accomplices in Nigeria, Libya and Sicily.

Police said the network ran an "extremely lucrative" operation. Telephone conversations between ring members and traffickers in Libya revealed large numbers of people hoping to be taken across the sea to Europe are being held captive near the Libyan capital Tripoli in appalling conditions.

"They are massed together to wait in captivity as price negotiations are completed and the payment is made (for the boat journey), during which time they are subjected to all types of abuse, from starvation to beatings to sexual assault," the police said in a statement.

Police said it began investigating the suspects in June after a Nigerian girl, a minor, informed officers that she had been smuggled to Italy and upon arrival was held captive and forced into prostitution. She had been held by the ring near the Sicilian city of Ragusa.

Among the record 181,000 all the migrants who reached Italy by boat last year, more than a fifth were Nigerian. A half-million migrants have arrived on Italians shores since the start of 2014.

Humanitarian groups say the number of illegal migrants being forced into prostitution in Italy is rising.

(Reporting by Steve Schere; Editing by Raissa Kasolowsky)