Motorists have been warned of "serious disruption throughout the day" as protesters target motorways in a demonstration over high fuel prices.
Demonstrations on the M4 and M5 in Wales and the South West took place early this morning, with "rolling roadblocks" being used.
This included vehicles going slowly both ways across the Prince of Wales Bridge between Wales and England, and a slow-moving procession on the M5 near Exeter heading northbound.
The procession on the bridge over the Severn stopped briefly, despite police saying vehicles cannot stop or travel slower than 30mph.
Roads across the country could see jams with Essex and Gloucestershire also expected to be targeted.
The protesters are targeting mostly three-lane motorways and result in slow-downs on two lanes, leaving the fast lane free, according to FairFuelUK founder Howard Cox.
Videos showed demonstrators slowing down all three lanes of traffic.
The protesters are understood to have been organised via social media under the banner Fuel Price Stand Against Tax.
'I lost two jobs because of fuel prices'
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."
For a few minutes, both carriageways of the M4 approaching the Severn crossing were brought to a standstill by go-slow protests travelling east and west.
Two police motorcyclists rode in front of four vehicles travelling at around 30mph, while a marked police patrol car drove behind the protesters, followed by dozens of queuing motorists.
A larger convoy of protesters drove over the Severn crossing heading into England from Wales with a large backlog of traffic following behind.
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.
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.
"The retailers came up with an excuse that demand had fallen to 80% for some," he said.
"Yet, last week, official statistics showed that petrol consumption is still at 94% of normal.
"That is incredible given the enormous pump-price pressure on drivers and underlines once again that road fuel is an essential expenditure for private car users and their families across the UK."
RAC fuel spokesman Simon Williams called on the large supermarkets that dominate fuel sales to explain their reasoning for keeping prices so high.
'Policing operation to limit disruption'
The government said although it understands people are struggling with rising prices and have a right to protest, "people's day-to-day lives should not be disrupted" and warned that traffic delays "will only add to fuel use".
Gwent Police Chief Superintendent Tom Harding said he would encourage drivers to reconsider their journey, consider working from home and avoid the area where possible.
Bristol Airport advised travellers to allow extra time for journeys.
A post on the airport's Twitter account said: "Please note that there is a planned fuel protest to block the River Severn Bridge crossings this Monday July 4 from 8.30am.
"The protest will likely affect the M5, M4 and the two crossings to Wales. Please allow extra time if travelling to or from the airport."
Essex Police Chief Inspector Anna Granger said her officers "are experienced at dealing with incidents which cause significant disruption".
Gloucestershire Police said protests will likely affect the A48, causing travel disruption in the Gloucester and Forest of Dean areas.
A government spokesperson said: "While we respect the right to protest, people's day-to-day lives should not be disrupted, especially on busy motorways where lives are put at risk and resulting traffic delays will only add to fuel use.
"The new Public Order Bill will make it a criminal offence to glue yourself to a dangerous motorway, which sees police spending hours trying to safely remove people."