Rolling roadblocks have brought parts of the M4 to a standstill as protesters target motorways in a demonstration over high fuel prices.
Police have warned of "serious disruption throughout the day" with organisers expected to block the Prince of Wales Bridge crossing between England and Wales.
Traffic Wales posted on social media saying there was a 'rolling roadblock' on the M4 eastbound from junction 23A in Magor to junction 23 in Rogiet.
Traffic is currently being directed off J23.
Protesters left the M4 Magor services, near Caldicot at 7:30am heading across the Prince of Wales bridge crossing the River Severn into England.
A convoy of around 20 vehicles left the services.
Before departing they were told by police they cannot stop and must drive no slower than 30mph with police officers planning on directing protesters off the motorway either side of the bridge.
Some protesters said they intended to meet in the middle and block the motorway.
Mobile welder Richard Dite, 44, from Maesteg said it is costing him over £300 in fuel to get to work every week due to price hikes.
"It's costing me £300 a week before I even get to work and earn anything," 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."
He was joined at Magor Service Station with around a dozen or more other people who have driven this morning across the Prince of Wales Bridge in protest of fuel tax.
Former HGV driver Vicky Stamper, 41, from Cwmbran said she and her partner Darren had to leave jobs in Bristol because they could not afford the fuel any longer.
Ms Stamper 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 said the situation had taken an emotional toll on her and her family.
Talking about the disruption that the protest will cause to drivers on the M4, Ms Stamper added: "We're doing this for us and for them. If they want to have a moan, they should join us instead."
Asked what she would ask Boris Johnson to do, she said: "Resign."
For a few minutes, both carriageways of the M4 approaching the M4 Prince of Wales Severn Bridge crossing were brought to a standstill by the go-slow protests travelling east and west.
Two police motorcyclists rode in front of four vehicles travelling at around 30mph from the Bristol area towards South Wales.
There was a marked police patrol car behind the protestors, followed by dozens of queuing motorists.
A larger convoy of protestors drove over the Severn crossing heading into England from Wales with a large backlog of traffic following behind.
Disruption is likely throughout the day. Protests will target mainly three-lane motorways and see slow-downs on two lanes, leaving the fast lane free, according to FairFuelUK founder Howard Cox.
While he said his organisation is not involved in the action, he is "fully supportive" of the demonstrations so long as they are conducted legally.
The protests are understood to be organised via social media under the banner Fuel Price Stand Against Tax.
Chief Superintendent Tom Harding, of Gwent Police, said: "We are seeing significant delays both east and westbound on the Prince of Wales Bridge due to the planned protest.
"We are seeking to return traffic to normal as soon as possible.
"Please keep an eye on our social media channels for further updates throughout the day."