French Interior Minister Bernard Cazeneuve, German Interior Minister Thomas de Maiziere, Tunisian Interior Minister Najem Gharsalli and British Home Secretary Theresa May lay flowers on June 29 at the site of the shooting attack in Sousse, Tunisia. (Kenzo Tribouillard/AFP/Getty Images)

Prime Minister David Cameron on Monday called the fight against the Islamic State “the struggle of our generation,” as Britain grappled with the aftermath of a beach massacre in Tunisia, the worst terrorist attack on British citizens since the London bombings on July 7, 2005.

Britain is in a state of mourning after a gunman on Friday sprayed bullets at European tourists lounging on a white beach in Sousse, killing at least 38. Eighteen of the victims have been confirmed as British, but officials say that number is expected to rise to around 30 as more bodies are identified.

The Islamic State, which is also known as ISIS or ISIL, has urged its supporters to attack the West during the holy month of Ramadan. There were also terrorist assaults Friday in France and Kuwait.

Speaking on BBC Radio 4’s Today program, Cameron said the radical Islamist group poses an “existential threat” to the British way of life and compared the fight against it to the Cold War. “It’s a battle of our values and our narrative against their values and their narrative,” he said. “We have to get that right as well as the military end.”

The rampage was the worst terrorist attack in recent memory in Tunisia, a country that has been widely hailed as a success story of the Arab Spring but that has suffered a number of deadly attacks by Islamist extremists.

Grieving tourists and locals held a candlelight vigil on the Tunisian beach where at least 38 people were killed in an attack. (Reuters)

On Monday, Tunisian officials announced they had arrested several people who allegedly helped the gunman, identified as 23-year-old student Seifeddine Rezgui. Interior Minister Najem Gharsalli did not identify them.

“We will find all those involved, whether it was just logistical support or not,” the Tunisian minister said, according to news reports.

Pictures and videos on social media have shown scenes of chaos as the black-clad gunman opened fire on his victims. He was later fatally shot by police.

Britain’s Scotland Yard has launched an investigation of the attack involving more than 600 officers and staff, one of the largest such operations since the London transit bombings of 2005. Some officers have been dispatched to Tunisia and others to British airports to interview travelers returning from Tunisia.

In Parliament, Cameron announced a national minute of silence to be held Friday at noon to remember those who lost their lives.

There has been frustration in some military circles that Britain isn’t doing more to fight the Islamic State in Syria and Iraq. Richard Dannatt, the former head of the British army, has argued that Britain should consider launching a ground operation in those countries because airstrikes have failed to stop the advance of the militants.

In an essay in the Daily Telegraph, Cameron called for a “full-spectrum response” to the attacks. Although it was unclear what exactly that would mean, he urged Britons to become “intolerant” of extremism and “stronger at standing up for our values.” He also said that security services must be given “the tools they need.” The prime minister has spoken previously about wanting to beef up security services’ powers to monitor online activity.

Theresa May, the British home secretary, flew Monday to Tunisia, where she and French and German ministers laid flowers on the Sousse beach.

“We are very clear that the terrorists will not win,” May said at a news conference with her German, French and Tunisian counterparts.

The government said all of the wounded victims will be returned to Britain within 24 hours aboard British military aircraft.

Meanwhile, tributes continued to pour in for the slain tourists. Trudy Jones, a 51-year-old divorced mother of four, was at the beach resort with her friends when she was slain.

“Our mother of all people didn’t deserve this, such a caring person who put everyone else before herself,” her family said in a statement issued by Gwent police, in Wales.

James and Ann McQuire, a recently retired couple from Cumbernauld, near Glasgow, Scotland, were named in their local church on Sunday as two of the victims.

Their friend Andrew Eadie struggled to hold back the tears as he told the BBC: “I understand there are freedom fighters and people trying to make points, but there must be other ways than what he’s done.”

Read more:

Witnesses recall the horror of terrorist attack on Tunisian beach

Why Tunisia, the Arab Spring’s sole success story, suffers from Islamist violence

Sousse: The beautiful Tunisian city that became a scene of horror