The Easter Sunday bombings in Sri Lanka had killed 258 people. (FILE)

Sri Lanka's president extended on Wednesday by a further month the state of emergency imposed immediately after the Easter Sunday bombings that killed 258 people.

Maithripala Sirisena issued a proclamation saying that the emergency, which gives sweeping powers to security forces to arrest and detain suspects for long periods of time, would continue for another 30 days, citing "public security".

Sri Lanka initially imposed the emergency to crack down on local terrorists blamed for the April 21 bombings that targeted three churches and three luxury hotels.

Three weeks after the suicide bombings, anti-Muslim riots broke out in a province north of the capital in a backlash against the attacks. At least one man was killed and hundreds of shops and homes were destroyed. Several mosques were also vandalised.

The police and the military say they have arrested scores of suspects, both in connection with the bombings and over what appeared to be organised violence against the Muslim minority.

The authorities say they have neutralised the terror threat after arresting almost all those involved in the Easter attacks, but troops and police remain on alert across the island.

Christians make up 7.6 percent and Muslims 10 percent of mainly Buddhist Sri Lanka.