Before many African migrants can even begin the perilous sea crossing to Europe they must pass through Libya, where they face exploitation or arrest by often brutal militias.

The feared drowning of some 300 African asylum-seekers in a shipwreck off Italy this week has underscored the dangers that desperate migrants from Sub-Saharan Africa face as they seek a better life in Europe.

But before many can even cast off, they must make their way across Libya, a vast desert country dominated by a patchwork of militias which emerged during the 2011 uprising that toppled Moamer Kadhafi.

On Friday, a group of African immigrants gathered at a church in Tripoli to mourn those who perished in Thursday's shipwreck, the worst-ever Mediterranean refugee boat disaster.

But they said it was unlikely to discourage future attempts.

"Despite the difficulties, the Africans here are not giving up on their dreams of going to Europe," said Albert Obmila, a Rwandan migrant.

"Life in Libya has become hell for Africans, who are at the mercy of the militias."

Under Kadhafi's regime, migrants risked arrest and deportation, but the myriad dangers posed by the militias are far worse, said George Ikbo, a Nigerian migrant.

He has been stuck in Libya for five years, in part because of "scams by Libyan smugglers in collusion with the militias," he said.

"Me too," Cameroonian migrant Patrick Adamo chimed in.

"I worked in all kinds of unimaginable trades and endured every hardship to save up 1,500 dinars (around $1,100)," the price of sea passage.

"There were more than 20 of us Africans who had paid the price," he said. But on the appointed day, "instead of putting us on the boat, they sent armed men from a militia to arrest us and detain us in Tripoli," where the migrants were held for four months.

"Since then I take every precaution to avoid falling into this trap while trying to make my way to Europe," Adamo said.

Several migrants spoke of similar scams, saying the heavily armed militias charge $1,000 to $2,000 per person for the crossing before having the migrants arrested at the last minute.

An Eritrean migrant interviewed by AFP, who asked not to be named, said he had been ripped off on three occasions.

Once detained, some migrants are handed over to immigration authorities, while others are jailed by the militias themselves.

Those unable to pay for their own release risk being loaded onto trucks and left at the desert borders of Chad and Niger. In June, Amnesty International estimated that some 25,000 migrants had been deported since May 2012.

Others remain in detention under "deplorable" conditions, according to the Amnesty report.

During the 2011 uprising, rebels often branded African migrants and even dark-skinned Libyans as pro-Kadhafi mercenaries because of the strongman's close relations with neighbouring African countries.

'Abuse risks becoming permanent feature'

In the June report, Amnesty International documented instances of ill-treatment amounting to torture, saying migrants, including women, had been beaten with water pipes and electrical cables while being detained at holding centres.

The group uncovered two instances of detainees having been shot by live fire during riots.

“The torture and ill-treatment we uncovered at ‘holding centres’ is unacceptable and is a stain on the record of post-Kadhafi Libya," Hassiba Hadj Sahraoui, Amnesty's Deputy Programme Director for the Middle East and North Africa, said in June.

"Abuse of Sub-Saharan foreign nationals was a hallmark of Kadhafi’s rule and risks becoming a permanent feature of the country if the Libyan authorities don’t immediately reverse their policies."

Libyan authorities have requested Western aid, saying their fledgling post-revolutionary government is not yet up to the task of patrolling the country's 4,000-kilometre (2,500-mile) land border with six states or its 1,700 kilometre coastline.

Kadhafi had made similar appeals for help in combatting illegal immigration, demanding five billion euros a year from the European Union.