TRIPOLI May 6 (Reuters) - Libya's coast guard detained on Wednesday almost 600 illegal African migrants, among them pregnant women and 18 children, who had tried to sail to Europe on a fishing boat, a security official said.

The North African country, gripped by violence and a breakdown of state authority four years after the ousting of Muammar Gaddafi, has turned into a major hub for human traffickers smuggling African migrants by boat to Italy.

As many as 900 people may have died last month off Libya's coast when their packed boat capsized as they were trying to reach the Italian island of Lampedusa. Many thousands more have managed to make the crossing in an almost daily exodus of boats.

"The coast guard arrested early this morning 585 illegal migrants from different African countries such as Somalia, Eritrea, Ghana and Mali," said Mohamed al-Ghwaili, a spokesman for a police department tasked with tackling immigration.

He said the Africans had tried to set off to Italy in an old fishing boat in Sabratha, a town west of the capital Tripoli.

Libya's coastguard, under pressure from Europe to stem the flow, said on Sunday they had intercepted several boats carrying around 500 migrants and brought them back to shore.

Captured immigrants are held in often cramped detention centers in Libya, sometimes for months on end, human rights groups say.

Libya's coast guards have few resources and rely on fishing and tug boats to combat human trafficking as the central state authority has collapsed across the oil producer, with competing governments and dozen of armed groups battling each other. (Reporting by Ahmed Elumami; additional reporting by Omar Fahmy in Cairo; Writing by Ulf Laessing; Editing by Crispian Balmer)