ISESCO said in a statement on Sunday that this heinous act is a crime against humanity and an ugly face of hatred and extremism, Xinhua news agency reported.
The attacks "hamper international efforts to promote the values of peace, coexistence and respect among followers of religions," it added.
Sri Lankan police said that 13 people had been arrested over a series of blasts which killed 228 people across the nation.
Sri Lankan police have arrested two dozen people in connection with a string of bomb attacks on several churches and hotels that have so far claimed the lives of at least 290 people.
The country went on lockdown on Easter Sunday after eight apparently coordinated blasts hit churches filled with worshipers and luxury hotels in the capital, Colombo, as well as in the cities of Negombo and Batticaloa.
Police announced the dramatic rise in the death toll on Monday morning, saying some 500 people had also been injured in the attacks, the worst of their kind since the end of Sri Lanka’s devastating civil war a decade ago.
There were 32 foreigners among those killed, including nationals from Britain, the US, Turkey, India, China, Denmark, Netherlands and Portugal.
Three police officers were also killed several hours after the attack, when they raided a house in Colombo. Police reported an explosion at the house.