If the feared death toll is confirmed the disaster will be the "worst catastrophe in recent years", Hollande told Canal+ TV.
The 28 survivors have indicated that about 700 people were on board the boats that went down near the Libyan coast.
Two other shipwrecks this week left an estimated 450 people dead.
Europe must act to end the mass drownings, Hollande commented and he called for the people who organise the voyages to be tracked down "because those who put these people on boats are traffickers, terrorists even, because they know these boats are lousy... and put hundreds of people in danger".
Later in the day the European Commission announced that a ministers' meeting would take place, although it did not give a date.
It will prepare a new European migration strategy to be adopted in mid-May, a statement said.
French opposition leader Nicolas Sarkozy called for the European Union to adopt a "real immigration policy to prevent these tragedies" in a tweet on Sunday afternoon.