At least six attacks took place targeting churches and hotels, and as of 2: 30 pm IST, a seventh attack was also reported by Reuters.
The Sri Lankan government has declared curfew with immediate effect and it is not clear when it will be lifted, Reuters reported. The decision came in the wake of a seventh blast that has reportedly killed at least two people in Dehiwala, near Colombo.
According to a Reuters report, access to major social media services and messaging services was also shut down.
Earlier, the US embassy in Colombo asked travellers to follow security instructions from local authorities and urged them to seek "shelter" and "exercise extreme caution.” The United Kingdom also issued a travel advisory.
According to the island nation's The Daily Mirror newspaper, explosions were reported as hundreds of worshipers gathered at the
Three blasts where reported at the Shangri-La, Cinnamon Grand and Kingsbury five-star hotels in Colombo.
An emergency meeting was called over the attacks, Minister Harsha De Silva said in a tweet.
"Secretary of Defence and I am at Kochchikade Church. We were also at Shangri-La and Kingsbury Hotels. Rescue operations are underway. Many casualties have been reported including foreigners," he said and requested people to stay calm and indoors.
Images on social media showed the inside at the St. Sebastian church with a shattered ceiling and blood on the pews, the BBC said.
No one has yet claimed responsibility for the attacks.
External Affairs Minister Sushma Swaraj said on Sunday that she was in touch with the Indian High Commissioner in Colombo as six explosions hit churches and hotels in Sri Lanka, killing at least 20 people.
"Colombo - I am in constant touch with Indian High Commissioner in Colombo. We are keeping a close watch on the situation," the Minister said in a tweet.
The official handle of the High Commission of India in Colombo also said in a tweet that they were also "closely monitoring the situation".
It also provided helpline numbers for "Indian citizens in need of assistance or help and for seeking clarification".
With agencies
See also:
Find shelter and exercise extreme caution, says US’ travel advisory after multiple blasts in Colombo, Sri Lanka
Colombo airport asks passengers to check in four hours before flights after terror attacks in Sri Lanka’s capital
Schools to stay shut in Colombo for two more days: Sri Lanka’s Education Minister