Protesters using “rolling roadblocks” have brought parts of the M4 to a standstill during demonstrations over high fuel prices.
It comes as police have said they are aware of driving offences taking place during motorway protests over the price of fuel.
And a reporter from the PA news agency saw protestors being arrested.
Demonstrations took place on the M4 at the border between England and Wales, and on the M5.
Groups of motorists took part in “slow downs” where they occupied all three lanes and dropped their speed to back up traffic behind them.
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Video footage also showed one driver holding his phone as he drove on the M4. Using a phone for any reason while behind the wheel is an offence.
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A spokesman for Gwent Police said: “We are aware of driving offences being committed during the planned protest on the M4.
“We are committed to increasing the safety of all road users in Gwent and beyond.
“We urge all motorists to drive carefully, responsibly and within the limits of the law.”
Drivers also came to a stop during the protests, despite warnings from police that they must maintain a minimum speed of 30mph.
The RAC says on its website: “Although there isn’t an official minimum speed limit on most motorways, travelling too slowly can be considered dangerous and you might attract the attention of the police.”
The protesters are understood to have been organised via social media under the banner Fuel Price Stand Against Tax.
HGV driver Vicky Stamper, 41, has lost two jobs due to high fuel prices.
Both her and her partner, from Cwmbran in South Wales, used to work in Bristol.
She said: “We had to leave those jobs because it was costing us £380 a week just to get to and from work.
“I then lost a job two weeks ago because the company couldn’t afford to put fuel in that many lorries so, last in first out.”
She also called for Boris Johnson to resign.
Richard Dite, 44, from Maesteg, South Wales, said it cost him more than £300 in fuel to work every week.
He said: “My only option soon will be to put the welding gear in the shed and call it a day, maybe go on the doll.
“Face it at this rate I’ll be on more that way.”
Retailers failing to pass on falling wholesale costs
Figures from data firm Experian show the average price of a litre of petrol at UK forecourts reached a new high of 191.5p on Sunday. Diesel hit 199.1p on Thursday before dropping down to 199.0p on Sunday.
The AA criticised retailers for failing to pass on nearly a month of falling wholesale costs.
Petrol wholesale costs have been down at least 5p per litre for more than a fortnight, having ended last week 10p down on record highs of early June, the AA said.
“It is an outrage, plain and simple, that the fuel trade could be slashing petrol prices as the nation heads towards the holiday season, but isn’t,” said Luke Bosdet, the AA’s fuel price spokesman.
Chancellor Rishi Sunak has said he will carefully consider calls for a “more substantial” fuel duty cut after the 5p per litre reduction implemented in March failed to halt price rises.
