Sri Lankan authorities have ordered a curfew in the capital, Colombo, for a second day on Monday.
It will be in place from 1430 GMT to 2230 GMT on Tuesday, the government information department said.
So far, the police have arrested 24 people and are carrying out searches across the island.
No group has claimed responsibility for the attacks so far.
The Interpol on Monday said it is ready to offer full support to the Sri Lankan authorities in investigating the attack.
This comes as Prime Minister Ranil Wickremesinghe told reporters that some officials had prior intelligence about the attacks but didn't act on it. "This is a matter we need to look into."

Trump adds seven countries to full travel ban list
Indian parliament votes to allow private firms in nuclear power sector
Doctors in England start five-day walkout during flu surge
Israeli settler kills 16-year-old Palestinian in West Bank, mayor says
Paris' Louvre reopens partially but staff vote to extend strike