The Queen and Duke of Edinburgh joined people across the country in pausing to remember the victims, 30 of whom were British holidaymakers.
The number of British fatalities, at the hands of Islamist gunman Seifeddine Rezgui, was the highest death toll this country has suffered in a terrorist attack since the London bombings 10 years ago, when 52 people were killed.
Three Irish people were also killed in last Friday’s attack, which was claimed by Islamic State, along with two Germans, one Portuguese, one Belgian and one Russian.
Prime Minister David Cameron marked the silence in his Oxfordshire constituency, with the government encouraging schools and local authorities to do the same.
Flags are being flown at half mast throughout the day over Buckingham Palace, Downing Street and other government buildings, as well as police stations, military bases and embassies around the world.
The start of play at Wimbledon was delayed by 45 minutes to 12.15pm on outer courts, to allow tennis players and fans and tournament staff to observe the silence.
Plans for a national silence were announced by Mr Cameron at Westminster on Monday, just moments after MPs had observed their own minute of silent memory in the Commons chamber.
He said there would also be discussions with victims’ families about a memorial to those killed in Sousse, where another ceremony was held today.
The first inquests into the deaths were opened today at West London Coroner’s Court. So far, the bodies of 17 victims have been returned to the UK. Eight others will arrive at RAF Brize Norton later today, with five on Saturday.
Rezgui targeted foreigners in his rampage at the Imperial Marhaba Hotel before he was shot dead by police. The Tunisian authorities say he was trained in Libya at a jihadist camp last year.
He is thought to have had accomplices who helped him to carry out the shootings and the Tunisians have made a number of arrests, with eight people in custody suspected of having direct links to the massacre.