The attack happened at the popular holiday resort of El Kantaoui, north of Sousse.

It came on a day when terrorists struck on three continents. In France, a lone killer decapitated a man at a liquid gas factory and flew a jihadist flag. In Kuwait, Islamic State of Iraq and the Levant claimed responsibility for a suicide bombing at a mosque that killed 27 and injured hundreds.

Witnesses said the gunman was laughing and joking as he selected his victims, and that he was specifically targeting British, French and other tourists.

German, Belgian and Irish tourists were among the dead and 36 injured after the gunman landed on the beach from a small rubber-hulled speedboat before spraying bullets into the crowds of sunbathers.

One victim was named as nurse Lorna Carty, from Ireland, was shot as she went to the beach before her flight home. The mother-of-two had taken her husband Declan on holiday as he recovered from heart surgery.

As well as the 16-year-old boy, who has yet to be formally identified, the wounded included Matthew James, 30, who was shot four times in the stomach while protecting his fiancee, Sarah Wilson.

"He took a bullet for me," she said outside the hospital. "I owe him my life because he threw himself in front of me when the shooting started. He was covered in blood from the shots but he just told me to run away.

"I ran back, past bodies on the beach to reach our hotel. It was chaos - there was a body in the hotel pool and it was just full of blood."

The assassin was named locally as Seifeddine Yacoubi, 23, an aviation student from the Tunisian city of Kairouan. The attack was the third of the day by militants believed to be loyal to or inspired by Islamic State.

In the morning, a man, named as Yassin Salhi, tried to detonate gas canisters at an industrial gas factory near Lyon in south-east France. A decapitated head, apparently that of his boss at the delivery firm where he worked, was found with Arabic writing and a black Isil-linked flag was attached to a fence nearby.

Salhi was said to have had no criminal record but to have been monitored for signs of "radicalisation".

Shortly afterwards, in Kuwait, a suicide bomber detonated his explosives vest in a Shia mosque packed with worshippers for Friday noon prayers.

The attack, which killed at least 27 people and injured more than 200, was claimed by the "Najd Province of Islamic State", a new branch of the terrorist group which sent suicide bombers into two Shia mosques in neighbouring Saudi Arabia last month. Najd is in central Saudi Arabia.

It is not clear whether the three attacks were connected or coordinated. But Islamic State's chief spokesman, Abu Mohammed al-Adnani, issued a tape warning on Tuesday that Ramadan, the Muslim holy fasting month which began last week, would bring "calamity for the kuffars [infidels]". There were also massacres by IS jihadists in the Syrian town of Kobane, and by the al-Qaeda-linked al-Shabaab movement in Somalia.

World leaders were united in outrage and promised further action to coordinate a response. David Cameron, the Prime Minister, summoned an immediate meeting of the Cobra emergency response committee.

"This is a threat that faces all of us," he said, at the end of the European Council summit in Brussels. "These events have taken place today in Tunisia and in France but they can happen anywhere. We all face this threat."

He said there should be a focus on improving counter-terrorism co-operation, but that taking on the radicalisation of young people was perhaps "more important". He added: "We have to combat it with everything we have."

In the United States, President Barack Obama was briefed before addressing a memorial service for the victims of the church shooting in Charleston, South Carolina, last week. "We stand with these nations as they respond to attacks on their soil today," the White House said.

Tunisia has become a key target for jihadist attacks. It is the one country that has undergone a relatively successful transition to democracy following the Arab Spring, and remained open to Western values. But at the same time it has contributed more foreign jihadists to the war in Syria than anywhere else.

In March, 21 people were killed in an attack on the Bardo Museum in Tunis, all of them tourists apart from a security officer and a cleaning lady.

The resort complex that was the scene of Friday's attack contains several hotels popular with British tour companies including Thomson, First Choice and Thomas Cook.

After shooting his way through the beach, the attacker made his way towards the hotels behind, before he was eventually shot dead by police. His body was photographed in the street, with a Kalashnikov assault rifle lying next to him.

There was confusion last night over the arrest of a man who was photographed being led away by armed police while a woman tried to assault him. It was not clear if he was connected to the attack.

In the wake of the attack, sun-loungers were used as makeshift stretchers to carry the wounded to ambulances.

Survivors told how they were hit while others were in hysterics as their wounded partners were taken to hospital. Frightened tourists barricaded themselves into their hotel rooms and a pregnant woman went into labour in the confusion.

Olivia Leathley, 24, heard "loud bangs" and saw from her hotel room that people were fleeing the beach, as holiday company representatives blew whistles.

"All of a sudden, from the level just below the lobby there was a huge sound of loads of machinegun fire and one of the reps just said 'run'. I was crying and we heard the machinegun fire and it was so loud and it seemed like it was just behind us, it seemed so close."

Ellie Makin, from Ripon in the UK, was on the right-hand edge of her hotel's section of beach. "All I saw was a gun and an umbrella being dropped," she told ITV. "Then he started firing to the right hand side of us. If he had fired to the left I don't know what would have happened, but we were very lucky."

Tomas O Riordain, from Cork, was in a neighbouring hotel. "It was all over three or four minutes after it started," he told RTE radio, saying he had two daughters at the beach and "just had to wait and see if they came back". It was the second time he had been caught up in a terror attack, as he was in Liverpool Street during the London 7/7 bombings.

The Telegraph, London