600 people are taking part in a march through Newport ahead of the Nato summit being held in city.
Police and council officials warn the march may cause disruptions as rolling road blocks could hit travel.
More than 150 heads of state and ministers will attend the summit.
A peace camp has been set up at the city’s Tredegar Park with more campaigners expected to arrive in the run up to the summit at Newport’s Celtic Manor Resort from Thursday.
Council highways officials said a number of roads will be affected due the march which started from Newport Crown Court car park at about 14:00 BST.
Groups including Stop the War have organised coaches with campaigners being driven in from London, Birmingham, Norwich and Newcastle.
The Campaign for Nuclear Disarmament (CND) has helped to organise a Counter Summit on Sunday at Cardiff County Hall with talks from political dissidents and peace activists.
Speaking before the protest started, Gwent Police Superintendent Alun Thomas said officers did not expect any trouble.
“We have been engaging with the protesters for a number of weeks, and we have signed memorandums of understanding with protesters and we anticipate a peaceful protest and we will seek to facilitate that in accordance with the law.
“If there were any problems we have sufficient resources to deal with those, but we do not anticipate any problems today.”
Newport City Council’s Labour councillors have sent an open letter to world leaders attending the summit calling for “political, not military, solutions” to the situation in Gaza amid hopes of a long-term truce between Israel and the Palestinians.
“We wish you well in all your endeavours to secure peace in the world,” they say.
But Pippa Bartolotti, Wales Green Party leader and one of the No Nato Newport organisers, said: “The Nato that so many people remember as a force for defence has turned into a force for aggression without anyone really being aware that this subtle change has happened over the last 10 to 15 years.”
Meanwhile in Cardiff, road closures will be in place on Saturday night through to Sunday morning to allow further work on a security cordon around Cardiff Castle and the Royal College of Music and Drama where summit dinners will take place.
Cardiff council said the east and north bound carriageway from Castle Street to North Road would be shut from 20.30-06.30 BST.