The coast guard said more than 1,300 people had been rescued on Wednesday from 13 makeshift boats.
At dawn on Thursday, the Aquarius, a humanitarian ship chartered by SOS Mediterranee and Doctors Without Borders (MSF), rescued 100 others, including seven women and 41 unaccompanied minors.
Picked up 22 nautical miles (41 km) off Sabrata, the rescued migrants – travelling with only an old compass to help them navigate and no lifejackets – said they had set off around 10:00pm along with other people.
“Those on board tell us there are other boats needing help so the crew is searching hard and fast,” MSF Sea said on Twitter.
An SOS Mediterranee spokesman told AFP “the sea is calm, it is 15 degrees (centigrade), we are continuing the search”.
Slightly further north, preparations were under way in Malta for a European summit on Friday which will be largely devoted to the worst migrant crisis since World War II and the fight against people traffickers in Libya.
Closing the smuggling route from Libya to Europe is “within our reach”, EU chief Donald Tusk said on Thursday.
But Doctors Without Borders retorted that “declaring war on people smugglers does nothing to end the wars, poverty and lack of alternatives that fuel their trade”.
On the Aquarius, the idea of blocking migrants in a country where they are victims of abuse and torture seems utterly inhumane.
“The Libyans shoot us like dogs,” Boubacar, a 17-year-old Guinean, said according to a SOS Mediterranee spokesman.
Italy says some 4,480 people were rescued and brought to the country in January, while the UN says over 220 people died or were lost at sea.