Political and religious leaders across the world have rushed to condemn the killing of nearly 300 people in a wave of bombings that targeted churches and luxury hotels in Sri Lanka on Easter Sunday.

The death toll from the attacks on three churches and three hotels on the island nation stands at 290 but is expected to rise, with at least 500 injured. Dozens of foreign visitors are among the casualties, with five British citizens confirmed killed.

Theresa May, the UK prime minister, called the blasts “appalling”, while Justin Welby, the archbishop of Canterbury, called for solidarity with “the people of Sri Lanka in prayer, condolence and solidarity as we reject all violence, all hatred and all division”.

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Donald Trump offered condolences to the people of Sri Lanka and said the US stood “ready to help”, while António Guterres, the United Nations secretary general, said he was “outraged by the terrorist attacks” and stressed the “sanctity of all places of worship”.

The six initial blasts appeared timed to cause maximum casualties among worshippers attending Easter services and customers in restaurants eating breakfast in the Shangri-La, Kingsbury and Cinnamon Grand hotels in Colombo.

The Sri Lankan security services are also likely to have questions to answer after Ranil Wickremesinghe, the prime minister, said there had been “information” about possible attacks, believed to be a reference to warnings reportedly received by local intelligence services around 10 days ago that “prominent churches” would be targeted by suicide bombers. It is not clear what if any precautions were taken.

A spokesperson for the National hospital said among the casualties were citizens of the US, Denmark, China, Japan, Pakistan, Morocco, India and Bangladesh. Eleven foreigners have been so far been confirmed dead, with nine missing. The toll is expected to rise.

The British MP Tulip Siddiq tweeted that she had lost a relative in the attacks but did not give any more details.

Facebook Twitter Pinterest A man walks across a deserted street during a curfew in Colombo. Photograph: Eranga Jayawardena/AP

There was no immediate claim of responsibility but the government said twenty-four people, all thought to be Sri Lankan, had been arrested and investigators would look into whether the attackers had “overseas links”.

Authorities also ordered a nationwide curfew and curbed social media access to restrict “wrong information” spreading throughout the country of 21 million people. Streets in the capital were deserted by early evening, with a heavy presence of security forces.

The numbers of casualties threatened to overwhelm medical facilities. At least 160 people injured in a blast at St Anthony’s Shrine in Colombo had been admitted to the Colombo National hospital by mid-morning, one official said. The main hospital in the eastern port city of Batticaloa had received more than 300 people with injuries following a blast at the Zion church.

Most of the bombings appeared to involve suicide attackers, officials said. One witness described a suicide bomber detonating his device when he reached the front of a buffet queue at the Shangri-La restaurant.

The bombings of the churches left casualties slumped among debris, damaged icons, and wrecked pews.

In St Sebastian’s in Negombo, near Sri Lanka’s international airport, more than 50 people were killed. Much of the church roof was blown out in the explosion, with roof tiles and splintered wood littering the floor and pools of blood in between wounded worshippers.

Two smaller explosions occurred three hours after the wave of bombings as security forces closed in on suspected attackers. Three police officers were reported to have died in one of the blasts.

The reported warnings, circulated by senior police officials, suggested that an extremist Islamist group called the National Thowheeth Jama’ath was behind the attacks. Wickremesinghe said the information had not been shared with him or cabinet ministers. “While this goes on we must also look into why adequate precautions were not taken,” he said.

The efficiency of security agencies in Sri Lanka has long been undermined by infighting between political factions and there has been no independent confirmation of the warning.

Facebook Twitter Pinterest Sri Lankan military stand guard near the explosion site in Batticaloa. Photograph: Reuters

Wickremesinghe said the government’s first priority would be to “apprehend the terrorists” and that investigators would look into whether the attackers had any “overseas links” though “so far the names that have come up are local”.

The blasts were the first major attack on the Indian Ocean island since the end of a bloody civil war 10 years ago.

Witnesses described distressing scenes at the sites of two attacks. “I saw many body parts strewn all over. Emergency crews are at all locations in full force,” Harsha de Silva, Sri Lanka’s economic reforms minister, tweeted after visiting the Shangri-La hotel and St Anthony’s Shrine. “We took multiple casualties to hospital. Hopefully saved many lives.”

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Colombo’s archbishop, Malcolm Ranjith, called on the public to rally in support of the victims, requesting all doctors to report to work despite the holiday and members of the public to donate blood. Crowds formed at clinics and hospitals as members of all Sri Lanka’s diverse ethnic and religious communities responded to his appeal.

About 1.2 million of Sri Lanka’s 21 million population is Catholic. More than two-thirds are Buddhists though there are sizeable Muslim and Hindu minorities.

The Muslim council of Sri Lanka issued a statement condemning the attack on the places of worship of “our Christian brothers and sisters on their holy day of Easter, as well as on the hotels in Colombo”. “We mourn the loss of innocent lives due to extremist and violent elements who wish to create divides between religious and ethnic groups to realise their agenda,” the statement said.

Facebook Twitter Pinterest The Sri Lankan prime minister, Ranil Wickremesinghe, speaks during a press conference in Colombo on Sunday. Photograph: Ishara S Kodikara/AFP/Getty Images

There has been growing intercommunal tension in Sri Lanka for several years. Last year, there were 86 verified incidents of discrimination, threats and violence against Christians, according to organisations that represent more than 200 churches and other Christian organisations.

This year, the National Christian Evangelical Alliance of Sri Lanka (NCEASL), recorded 26 such incidents, including one in which Buddhist monks allegedly attempted to disrupt a Sunday worship service, with the last one reported on 25 March.

It was unclear how long the ban on social media would last. Government ministers urged people not to foster “racial disharmony”. In a statement, Udaya R Seneviratne, from the office of the president, said the government had “taken steps to temporarily block all social media avenues until the investigations are concluded”.

The government moved to ban Facebook and other sites last year after blaming reports of an attack on a Buddhist temple for deadly anti-Muslim riots by hardline Buddhist groups.

Vladimir Putin, Emmanuel Macron and other leaders condemned the attacks. So, too, did Yousef al-Othaimeen, the secretary general of the Organization of Islamic Cooperation, who described them as “cowardly attacks” that targeted innocent worshippers and civilians. Fifty-seven nations are part of the OIC, headquartered in Jeddah, Saudi Arabia.

Pope Francis, giving his Easter sermon at the Vatican, expressed his “heartfelt closeness to the Christian community [of Sri Lanka], wounded as it was gathered in prayer, and to all the victims of such cruel violence”.