Easter Day bomb blasts at three Sri Lankan churches and four hotels killed 310 people and wounded around 500, following a lull in major attacks since the end of the civil war 10 years ago.
The explosions, some of which officials said were suicide bomb attacks, led to an immediate clampdown, with the government declaring a curfew and blocking access to most major social media and messaging sites.
The powerful blasts - six in quick succession and then two more hours later - wrought devastation, including at the capital's well-known St Anthony's Shrine, a historic Catholic Church.
The three hotels hit in the initial attacks were the Shangri-La Colombo, Kingsbury Hotel in Colombo and the Cinnamon Grand Colombo.
The first six explosions were all reported within a short period in the morning just as church services were starting. Hours later there were two further attacks in the outskirts of Colombo.
Police the attacks were carried out by suicide bombers.
Who were the victims?
The death toll rose to 310 on Tuesday after several people died of their injuries overnight, a police spokesman said. There were hundreds of people injured in hospitals.
There were eight British citizens killed in the attack, two of whom had dual US nationality.
Ben Nicholson said his wife Anita, 42, son Alex, 14, and daughter Annabel, 11, had been killed as they sat at a table for breakfast in the Shangri-la Hotel in Colombo on Easter Sunday.
"Mercifully, all three of them died instantly and with no pain or suffering," Mr Nicholson said.
Daniel Linsey, 19 and his younger sister Amelie, 15, were having breakfast with their father Matthew at the luxury Shangri-La Hotel when the suicide bomber struck.
Dr Sally Bradley and her husband Bill Harrop were staying in the Cinnamon Grand Hotel when one of the seven suicide bombers struck.
The Manchester couple had been living in the Australian city of Perth since 2013 where Dr Bradley was practising medicine, but were due to return to the UK soon.
Tulip Siddiq, the Labour MP for Hampstead and Kilburn, said she lost a relative in the attacks. "It is all so devastating," she wrote on Twitter. "Solidarity with the people of Sri Lanka."
The first American victim of the Sri Lanka terror attack has been named as 40-year-old Dieter Kowalski.
Mr Kowalski, from Denver, Colorado, checked into the luxury Cinnamon Grand Hotel in Colombo just hours before it was targeted by the bombers.
Sri Lanka’s foreign ministry said the nationalities of 11 foreigners killed in the Easter Sunday blasts have been verified. Three Indians, one Portuguese and two Turkish nationals were killed, while a further nine foreigners were also reported missing. A Dutch national and a Chinese national also have been reported among the victims.
Who was behind the attacks?
A police spokesman said on Tuesday 40 people were now under arrest in connection with the attacks.
A Sri Lankan government official said the attacks were carried out by seven suicide bombers from a domestic militant group named National Thowfeek Jamaath. There was no claim of responsibility on Monday.
All of the bombers were Sri Lankan citizens, but authorities suspect foreign links, Health Minister Rajitha Senaratne said at a news conference.
Earlier, Ariyananda Welianga, a government forensic crime investigator, said an analysis of the attackers' body parts made clear that they were suicide bombers. He said most of the attacks were carried out by a single bomber, with two at Colombo's Shangri-La Hotel.
Documents seen by AFP show that Sri Lanka's police chief Pujuth Jayasundara issued an intelligence alert to top officers 10 days ago, warning that suicide bombers planned to hit "prominent churches".
"A foreign intelligence agency has reported that the NTJ (National Thowheeth Jama'ath) is planning to carry out suicide attacks targeting prominent churches as well as the Indian high commission in Colombo," the alert said.
The NTJ is a radical Muslim group in Sri Lanka that was linked last year to the vandalism of Buddhist statues.
Sri Lankan police were holding a Syrian national in custody for questioning, three government and military sources told Reuters on Tuesday.
"The terrorist investigation division of the police arrested a Syrian national following the attacks for interrogation," a source said. Two other officials with knowledge of the investigation confirmed the detention. "He was arrested after interrogation of local suspects," a second source said.
Key intelligence on a possible terrorist attack was not passed onto the Sri Lankan government weeks before the attack.
Prime Minister Ranil Wickremesinghe acknowledged late on Sunday that “information was there” about possible attacks, adding that “we must also look into why adequate precautions were not taken.”
How did Sri Lanka react?
The government beefed up security and imposed an immediate and indefinite curfew across the country.
It also put in place a "temporary" ban on social media platforms "in order to prevent incorrect and wrong information being spread".
Security at Colombo's airport was also enhanced, according to Sri Lankan Airlines, which advised its passengers to arrive four hours before their flights. It added that passengers with passports and tickets will be able to reach the airport during the curfew.
The Archbishop of Colombo, Cardinal Malcolm Ranjith, called on Sri Lanka's government to "mercilessly" punish those responsible "because only animals can behave like that."
Two Muslim groups in Sri Lanka also condemned the church attacks.
The Muslim Council of Sri Lanka said it mourned the loss of innocent people in the blasts by extremists who seek to divide religious and ethnic groups.
The All Ceylon Jammiyyathul Ulama a body of Muslim clerics, said targeting Christian places of worship cannot be accepted.
Embassies in Sri Lanka have warned their citizens to shelter in place.