The government has blamed a little-known local jihadist group, National Thowheed Jamath, although no-one has yet admitted carrying out the bombings.
Another 500 people were injured in the suicide attacks on churches and hotels, the BBC reported.
Police arrested 24 people in a series of raids and the president’s office declared a state of national emergency.
The emergency declaration, which comes into effect from midnight (18:30 GMT) on Monday, will give police and military extensive powers to detain and interrogate suspects without court orders.
On Monday, another blast rocked a street near a church in the capital, Colombo. Police were attempting to defuse explosives in a vehicle used by the attackers when it blew up.
Sri Lankan authorities were warned about a bomb threat from National Thowheed Jamath a full two weeks before the attacks, Cabinet spokesman Rajitha Senaratne said at a press conference.
He said that the warnings were not passed on to the Prime Minister, Ranil Wickremesinghe, or his cabinet. Wickremesinghe acknowledged that security services had been “aware of information” but had not acted on the information.
Defence Secretary Hemasiri Fernando said that the intelligence “never indicated it was going to be an attack of this magnitude”.
“They were talking about isolated, one or two incidents. Not like this,” he said.
He said “all important departments of the police” were informed about the warning, but acknowledged that no action was taken.
Senaratne said that authorities believed the bombers had international support. “We do not believe these attacks were carried out by a group of people who were confined to this country,” he said, adding that there was an international network without which these attacks could not have succeeded.”
A later statement said President Maithripala Sirisena would ask for foreign help to track down the international links to the attackers.
“The intelligence reports [indicate] that foreign terrorist organisations are behind the local terrorists. Therefore, the president is to seek the assistance of the foreign countries,” his office said.