According to previous reports, 321 people were killed in the explosions.
Police spokesman Ruwan Gunasekara has reportedly said Wednesday morning that 18 suspects were arrested overnight, raising the total detained to 58.
Sri Lanka's prime minister Ranil Wickremesinghe warned on Tuesday that several suspects armed with explosives were still at large, according to Reuters.
Earlier it was reported that around 500 people were wounded in the blasts, adding that 40 people were under arrest in connection with the attacks, which Sri Lanka's government has blamed on the local Islamist group National Thowheeth Jamaath.
A massive security operation was launched following the attacks. Sri Lanka's authorities introduced and re-introduced curfews, deploying an extra 1,000 troops in the capital Colombo.
Daesh* has reportedly claimed responsibility for the Sri Lanka attacks and released images that purported to show the attackers. Prime Minister Ranil Wickremesinghe said that investigators were still determining the extent of the bombers' foreign links, Reuters reported.
*Daesh (ISIS/ISIL/Islamic State), a terrorist group outlawed in Russia and numerous other countries