Libyan coastguards say they intercepted boats carrying 850 migrants trying to reach Europe on Sunday.
A spokesman said the migrants from various African countries – including 79 women, 11 of them pregnant – were found in seven inflatable boats near Sabratha, west of the capital Tripoli.
More than 30,000 migrants have already crossed from Libya to Italy this year.
It comes as the UN’s first World Humanitarian Summit is due to open in Istanbul.
Heads of state, aid agencies and others will discuss the financial response to crises and how better to distribute aid to those who most need it.
The UN says there is a $15bn annual gap in humanitarian funding and money raised is often held up at source.
However, the charity Medecins sans Frontieres has refused to attend, arguing the summit won’t pressurise states that violate humanitarian law and that no significant new commitments will be made.
Better weather conditions
Libya has become a major hub for migrants as people traffickers exploit lawlessness in the country.
The onset of better weather conditions has raised fears more people will attempt the dangerous 300 km (185 miles) crossing to the island of Lampedusa in Italy.
In March, French Defence Minister Yves Le Drian said there were about 800,000 migrants waiting to head to Europe from Libya.
Most are believed to be from Somalia, Sudan and Eritrea.
Libya has warned that it does not have the resources to control the flow of people heading to Europe and has accused the EU of failing to deliver on promises of help.
Earlier this month, a British parliamentary report said the EU’s naval mission to combat people trafficking off the Libyan coast was “failing” and had only succeeded in forcing smugglers to change tactics.
A note on terminology: The BBC uses the term migrant to refer to all people on the move who have yet to complete the legal process of claiming asylum. This group includes people fleeing war-torn countries such as Syria, who are likely to be granted refugee status, as well as people who are seeking jobs and better lives, who governments are likely to rule are economic migrants.