Campaigners have been prosecuted for inconsiderate driving while trying to bring roads to a standstill as part of a pro-Brexit protest.
The demonstrations aimed to ensure the UK leaves the European Union on 29 March by causing gridlock on motorways and A roads using a convoy of slow-moving vehicles, organisers said.
The protesters were aiming to target between 30 and 40 locations over the weekend, including the M25, M6 and M1.
Devon and Cornwall Police said it had stopped the convoys on the A30 and M5 and prosecuted the two lead drivers of both convoys for careless and inconsiderate driving.
The force said it had spoken to organisers of both protests beforehand and told them they were to ensure other motorists could pass them and not drop down to unsafe speeds.
Inspector Simon Jenkinson said that the force was "happy to facilitate" the protests as long as they did not bring the roads to a standstill.
But he added that the campaigners had blocked both lanes of the A30 towards Cornwall and at least two lanes of the M5 northbound while travelling at speeds as low as 20mph on the motorway.
"We took the decision for the front two vehicles to be pulled over and reported for the driving offence of careless and inconsiderate driving," he said. "They presented a risk to the road-using community. The information I have had was there were speeds as low as 20mph (on the M5). That presents a significant risk on a very busy arterial road."
The RAC advises that, while most motorways in the UK do not have an official minimum speed limit, "travelling too slowly can be considered dangerous" and might attract attention from police.
Highways England tweeted: "There have been a few issues but nothing of any major impact and at present everything is running as usual."
Organiser Ian Charlesworth had said the protests could cause "serious gridlock" and believes MPs and the Home Office "will be looking at it", but added he does not know how effective the protests will be.
"The ultimate aim is to make sure come hell or high water that Britain leaves on March 29," the 55-year-old said.
The protests have been organised through social media, with Mr Charlesworth's Facebook group containing more than 21,000 members.
After this weekend's protests he said a larger "national event" is planned in London on March 29, should their demands not be met, which will coincide with the pro-Brexit March To Leave demonstration in Parliament Square.
The Independent is however, supporting the Put it to the People march in London today.