Tunisian Interior minister Mohamed Najem Gharsalli, center, is flanked by British Home Secretary Theresa May, right, and German Interior Minister Thomas de Maiziere as they pay respect to the victims of Friday's shooting attack in front of the Imperial Marhaba hotel in the Mediterranean resort of Sousse, Tunisia, Monday, June 29, 2015. (Photo: AP)
Tunisia said Monday it had made its first arrests after a beach massacre that killed 38 people, as European officials paid tribute to victims of the country's worst jihadist attack.
British Home Secretary Theresa May, speaking at the scene of Friday's gun attack at a Tunisian holiday resort, vowed that "the terrorists will not win" after London warned that Britain's death toll could rise to "around 30".
The massacre, claimed by the Islamic State group, was the deadliest for Britain since the 2005 London bombings, and there are fears it could inflict a devastating blow to Tunisia's vital tourism industry.
Interior Minister Hajem Gharsalli said the authorities had arrested "a significant number of people from the network that was behind this terrorist criminal", referring to the lone gunman.
May travelled on Monday to the resort of Port el Kantaoui south of Tunis, and promised to fight extremism in the wake of the attack.
"We will be united in working together to defeat them but united also in working to defend our values," May said at a joint news conference with her German, French and Tunisian counterparts after visiting the scene of the killings.
"We are resolved... to defeat those who would do us harm, to defeat those who would undermine our freedom and democracy and to ensure that the terrorists do not win," she added.
May said there was "no evidence" to suggest the British casualties were targeted because of their nationality.
May and the German and French interior ministers, Thomas de Maiziere and Bernard Cazeneuve, joined Tunisian officials in laying a wreath in the sand near the Riu Imperial Marhaba Hotel where the attack happened.
Cazeneuve echoed May, promising that "we will win this war" and adding that France would help "ensure the development" of Tunisia's fragile economy.
A spokesperson for British Prime Minister David Cameron, meanwhile, said Britain had identified 18 of its nationals killed, but warned that the number may rise to "around 30".
Tunisia says four other victims have been identified as tourists from Germany, Portugal, Ireland and Belgium. Ireland said Sunday three of its citizens were killed.
Shocking new amateur footage from the attack has emerged on social media, showing the gunman walking calmly along the shore and bloodied bodies on the sand.
Intermittent gunfire can be heard in the 11-minute amateur video, recorded by a Tunisian man using his mobile phone who can be heard asking: "Why do you kill people? Why?"
The attacker, identified as 23-year-old student Seifeddine Rezgui, pulled a Kalashnikov assault rifle from inside a beach umbrella and opened fire on holidaymakers at the resort before being shot dead.
Gharsalli warned "anyone who provided any logistical or financial assistance" to Rezgui would be arrested.
"I promise the victims... that these criminal killers will be brought before Tunisian justice so they are justly punished," he added.
On Monday, handwritten messages could be seen placed next to the flowers on the beach reading "We are sorry" and "We are Muslims, not terrorists".
Cameron vowed to mount a full investigation and called for "a response at home and abroad" to violent Islamic fundamentalism.
The attack -- the second on tourists in Tunisia after the National Bardo Museum killings killed 22 people in March -- prompted authorities to boost security at attractions and along its 1,000 kilometres (620 miles) of coastline.
Officials have said they will deploy 1,000 armed officers from July 1 to reinforce the tourism police who will also carry guns for the first time, and there are plans to close 80 mosques accused of inciting extremism.
The shooting wounded 39 people, six of whom were still in "serious condition", the hotel's Spanish management said on Sunday.
Witnesses say the attack lasted 30 to 40 minutes, raising questions as to why security forces did not intervene sooner.
However, the interior ministry said police arrived "seven to eight minutes" after the shooting began.
Tunisia has struggled to deal with a rise in extremism since the 2011 revolution that ousted longtime strongman Zine El Abidine Ben Ali.
The recent attacks are damaging to the crucial tourism industry, which accounts for seven percent of Tunisia's Gross Domestic Product and almost 400,000 jobs.
Hundreds of tourists were flown home after Friday's attack but some have chosen to stay, among them Britons John and Lesley Edwards in Port el Kantaoui.
"It's a lovely place and people are great. We didn't want to leave, we feel pretty safe with the police and the army. There is more police now," John Edwards said.
"We feel sorry for the staff. We stayed mainly for them. Even if our family calls us every day to tell us to come back."