LONDON (Reuters/AFP) - Britain's Queen Elizabeth II and Prime Minister David Cameron led a nationwide minute's silence on Friday, a week after a militant gun massacre in Tunisia in which 30 out of the 38 victims were Britons.

Millions of people across Britain held the a minute's silence on an official day of mourning to pay tribute to the Britons killed a week ago, the country's worst loss of life in a militant attack in a decade.

The Britons were among 38 killed when Saif Rezgui opened fire on holidaymakers at the resort of Sousse in the North African country.

Flags on government buildings, British embassies around the world and at Buckingham Palace in London flew at half mast while at airports, train stations and across the country, millions of Britons fell silent at midday in memory of those killed.

Organisers at the Wimbledon tennis tournament delayed the start of play to allow spectators and those in the grounds to pay their respects, while other sporting events including the British Grand Prix were also marking the day.

The start of Wimbledon matches were delayed by 45 minutes to 12.15pm GMT to allow spectators and tennis players to take part.

In Tunisia, the British Embassy was planning to hold a memorial ceremony and lay wreaths on the beach where the attack took place.

Mourners held a minute's silence at the Tunisian beach resort that was the scene of the attack, an AFP journalist said.

The ceremony was organised by the British embassy and attended by Prime Minister Habib Essid, and was attended by the ministers of foreign affairs, tourism, health and youth and sports, flanked by a sizeable police presence.

Some tourists, yet to return home after last week's massacre, laid flowers in the sand as a plaque commemorating the dead was unveiled.

Last Friday's attack saw the highest number of Britons killed in a militant incident since four young British Muslims carried out suicide bomb attacks on London's transport network on July 7, 2005, killing themselves and 52 others.

Employees of travel group TUI, which includes operators Thomson and First Choice that organised the holidays of all of the British victims, stood in silence outside the company's headquarters.

There was also a ceremony outside Walsall football stadium in central England in tribute to three local men from the same family who died in the tragedy.

The moment of remembrance for the attack claimed by the Islamic State in Iraq and Syria (ISIS) comes a day after the British government raised the possibility of extending air strikes against ISIS militants from Iraq to Syria.

The Queen and her husband Prince Philip joined in the silence during a visit to Strathclyde University in Glasgow, while Mr Cameron marked the moment in his Witney constituency northwest of London.

The profile picture on the Prime Minister's Twitter account was changed to a sign reading "Remember Tunisia", with the first word written in red.

Britain has launched an investigation into the killings and the police said that they had so far taken 275 witness accounts and that more than 1,200 potential witnesses had returned to Britain.

The killings were the worst-ever massacre in Tunisia, which fears massive damage to its tourism industry.

The sector accounts for about 7 per cent of gross domestic product in a country already suffering from the upheaval that followed the 2011 overthrow of dictator Zine El Abidine Ben Ali.

St Paul's Cathedral joined in the remembrance and the Muslim Council of Britain also urged imams to deliver a sermon of peace at Friday prayers.

The bodies of 17 of the British victims have now been repatriated on a military transport plane to Royal Air Force base Brize Norton.

Eight more were due to be brought back to Britain later on Friday and the final five will return on Saturday.

The remains will be released to the families following post-mortem examinations.

Inquests into each of the 30 deaths will be opened to probe the circumstances of each death.

Three Irish nationals, two Germans, one Belgian, one Portuguese and one Russian were also among the dead.

The attack also comes as Britain prepares to commemorate the 10th anniversary of the July 7, 2005 attacks.

Tunisia on Thursday said eight people had been arrested in connection with the massacre by Rezgui, 23, who gunned down foreign tourists after pulling a Kalashnikov assault rifle from a beach umbrella.

Friday's attack was the second on tourists in Tunisia claimed by ISIS in three months, after the extremist group said it was behind a March attack on the National Bardo Museum in Tunis that killed 22 people.

Tunisian authorities have said Rezgui received weapons training from jihadists in neighbouring Libya, travelling to the chaos-wracked country at the same time as the two young Tunisians behind the Bardo attack.

In the past four years, dozens of police and soldiers have been killed in Tunisia in clashes and ambushes attributed to jihadists - mainly in the western Chaambi Mountains.

Disillusionment and social exclusion have fuelled radicalism among young Tunisians, with the country exporting some 3,000 jihadist fighters to Iraq, Syria and Libya.