The Dutch-flagged Sea-Watch 3, operated by German charity Sea-Watch, has been at sea for more than two weeks with 40 rescued Africans on board.
After waiting in international waters for an invitation from Italy or an EU state to accept the ship, captain Carola Rackete decided this week to sail for the southern Italian island of Lampedusa but was blocked by Italian government vessels.
But in the early hours of June 29, Rackete entered the port where the ship docked amid a heavy police presence.
Live television video showed Rackete being taken off the Sea Watch by tax police and driven away.
RAI state TV said she had been arrested for “resisting a war ship” which, RAI said, carries a penalty of up to 10 years in prison.
The migrants remain on board the ship.
Matteo Salvini, Italy’s interior minister and head of the right-wing League party, had previously said he would only allow Rackete to dock when other EU states agree to immediately take the migrants.
Even then, he said, Italian authorities would seize the ship and prosecute its captain for assisting people-smugglers, he said.
Premier Giuseppe Conte told reporters at the G20 summit in Osaka on June 28 three or four European countries were willing to take part in the redistribution of the migrants.
Rackete is already under investigation for breaking Italy’s beefed up laws against non-government rescue ships.
By Stephen Jewkes