Media playback is unsupported on your device
Police and council officers are patrolling the seafront in Bournemouth and Poole as thousands descend on the towns’ beaches.
By 09:00 BST roads were gridlocked and a mobile app to aid social distancing on the beaches was showing red in three areas – meaning “avoid”.
In June, a major incident was declared in Bournemouth because of the crowds.
In Kent, Thanet Council warned many of its most popular beaches were also nearing capacity.
Dorset Police said its officers would be working longer shifts and some had their rest days cancelled to cope with the anticipated influx of visitors over the summer.
By 11:00 traffic marshals were already turning people away from Sandbanks car park and residents were urging the council to close the road.
Yvonne Jones said local people would suffer as a result of the crowds
Yvonne Jones, 66, who has been a beach hut holder in Poole for 35 years, said: “There shouldn’t be this many people here – but Boris [the prime minister] should have made sure people stayed in their own counties.
“The traffic is horrendous – I live locally and it was a nightmare to get here. It’s the locals who end up suffering.”
Bournemouth Christchurch and Poole (BCP) Council leader Vikki Slade said the towns were ready to welcome visitors “with open arms” but had deployed marshals at car parks as well as extra security officers, first aiders and additional traffic management systems.
At the scene: Toby Wadey, BBC South
Traffic is winding every way you look on the roads approaching Sandbanks.
I watched as some flummoxed drivers finally made it to the car park entrance – only to be told in no uncertain terms to turn around. They left frustrated.
For those who had made it to the beach, they found the sand much busier than an average Friday.
On the whole though, the people I spoke to felt beach-goers were doing their best to be responsible – their biggest worry was what mess would be left behind to clear in the morning.
Motorists endured long queues to Sandbanks where the car parks were full
During the major incident on 25 June, police reported fights, overnight camping and three men were stabbed in an attack near Bournemouth pier.
Cars were left abandoned on verges as visitors struggled to park and the beaches were strewn with litter.
BCP Council said it now had powers to clamp or tow away vehicles parked illegally.
South East Trains warned social distancing was not possible on its coastbound trains
Southeastern Railway said trains bound for the Kent coast via Faversham were very busy making social distancing impossible.
Thanet Council urged people thinking of heading to Joss Bay, Botany Bay and Viking Bay in Broadstairs to reconsider their plans as the beaches there were nearing capacity.
Sunseekers were also warned to avoid Camber Sands in East Sussex where Rother District Council said the car parks were full.