Cycling fans are expected to line the 109-mile first stage of the Tour of Britain as Geraint Thomas rides in Wales for the first time as Tour de France champion.
The race started at Pembrey in Carmarthenshire and features four-time Tour de France winner Chris Froome.
A rolling road block will be in place on the route and organisers have warned that nearby roads “will be busy”.
The race is due to finish in Newport at about 15:00 BST.
Thomas has already had a formal homecoming after becoming the first Welshman to win the Tour de France earlier this year when 8,000 fans lined the streets of Cardiff.
Now the double Olympic track champion and multiple Grand Tour winner Froome star for Team Sky on the roads around south Wales.
They both warmed up for the race on Saturday by training with their Team Sky teammates and about 175 children in Maindy, Cardiff – where Thomas got the cycling bug as a child riding with the Maindy Flyers Youth Cycling Club.
The young cyclists got to ride with their heroes, including Froome.
Spectators hoping to see them in the Tour of Britain were advised to arrive at their preferred location about one hour and 30 minutes before the riders are due to pass.
After leaving Pembrey Country Park at 11:00 BST, the route includes three intermediary sprints in Carmarthen at about 11:40, Llandovery at about 12:40 and Usk at about 14:30.
The stage will finish outside the University of South Wales’ Newport city campus on Usk Way so the car park at the nearby Friars Walk Shopping Centre will be shut.
Organisers have warned fans taking selfies “that turning your back on the race is never a good idea” as riders pass “very fast”.
“We encourage everyone to get behind the riders, but please remember this is their race,” race chiefs said in a statement.
“Don’t block their route, run alongside them or impede their progress.”
Mark Cavendish, who has won 30 Tour de France stages, and German multiple Tour de France stage-winner Marcel Kittel will both miss this year’s eight-stage Tour of Britain.
Former world champion sprinter and Olympic medallist Cavendish, who won a Tour of Britain stage in Wales in 2013 when the fourth leg ended in Llanberis, is ill.
But multiple Grand Tour stage-winners Andre Greipel of Germany and Slovenia’s Primoz Roglic, who finished fourth at this year’s Tour de France, will compete.
Although a Tour of Britain stage has passed through Wales in each of the last eight years and ended in Cardiff in 2017, this will be the first time the tour has started in Wales since the race began in 1945.