A 'go slow' protest held by pro-Brexit campaigners around the UK has come to an end after delays on the M4 and the A55.
Police warned those involved could face prosecution, and thanked the public for "their patience".
Organisers planned to use a convoy of slow-moving vehicles on the M4 near Magor and A494 on Friday evening.
The protest is expected to continue on the A55 in Flintshire on Saturday.
Chief Superintendent Nigel Harrison of North Wales Police said: “North Wales Police are aware of two planned protests on the road network this coming weekend and we are in close and regular contact with the organiser."
“We have a legal duty to facilitate a peaceful protest, maintain public order, ensure public safety and uphold the law. We will of course balance all of this against the rights of those who wish to go about their normal business. We will take action to ensure public safety is not compromised and as such we are monitoring the situation and are in close contact with all partner agencies.”
Gwent Police said it is aware of the protest and working with organisers.
Organiser Ian Charlesworth, from Flintshire said the protests could cause "serious gridlock" but said he does not know how effective the protests will be.
He added he has "no worries at all legally" because "you have a legal right in this country to peaceful protest".
A Welsh Government spokesperson for Traffic Wales said the safety of road users is a priority.
Organisers are also planning a blockade at Holyhead Ferry terminal on Saturday 22nd March on Anglesey.
In Pembrokeshire, protesters are also planning to block lorries coming out of the ferry terminal on Saturday morning.
A larger national event and protest is planned in London on March 29.