An aid convoy made up of around 250 vehicles is being held at the port of Dover after being refused entry to France by authorities.
The convoy, organised by a number of campaign groups including the People's Assembly and Stop the War Coalition, left London on Saturday morning before being prevented from boarding ferries to Calais by French authorities.
The lorries, cars and minibuses are carrying aid donations for refugees and migrants living in the Jungle camp in Calais.
Pictures and video from the port show an impromptu rally, with people chanting and holding signs saying "refugees welcome" and "solidarity with refugees".
Earlier shadow chancellor John McDonnell tweeted in support of the convoy, posting: "Sending solidarity to @pplsassembly #ConvoytoCalais. As @HackneyAbbott said 'a beacon of hope at the end of a dark week'."
Spokesman for the People's Assembly Steve Sweeney said the convoy was separated from the rest of ferry traffic when they arrived in Dover.
He said: "It seems like a concerted effort stopping us from making the crossing. All the vehicles have been moved into a separate lane, it's incredible.
"A few cars have made it to France but they are being stopped there apparently."
He added they had been told it was "unlikely" they would be allowed to cross.
Asked on what grounds they had been stopped from travelling to Calais, Mr Sweeney said they had been told it was to do with heightened security in France.
He said they had been organising the convoy for around six months and accused Kent Police of colluding with their French counterparts to prevent them from crossing.
"The Kent police were taking down our number plates when we stopped on the way here. We're now holding a rally."
A spokesman for Kent Police said: "Kent Police has received notification from the French authorities that they have taken a decision to refuse the convoy entry to France.
"No agency within the United Kingdom has any grounds to challenge this decision. The refusal of entry to France is a matter for the French authorities.
"Kent Police is working with partner agencies in planning a policing response to minimise any disruption to the community, businesses and the public."
A port official at Dover said there was some "temporary disruption and delays" to services caused by the protest.
In a statement the prefecture of Pas-de-Calais cited a number of reasons for issuing the ban, including the potential for it to "generate violent episodes" and the lack of police officers available because of the ongoing European Championships.
They also said the numbers of vehicles involved may "facilitate the intrusion of migrants".
It added: "In these circumstances, the Prefect of Pas-de-Calais considered only the prohibition of any event is likely to effectively prevent public disorders that may occur.
"Moreover, any convoy on roads and highways, from Great Britain to Calais to slow or block the flow and constituting a public event, is prohibited."
Mr Sweeney later said the convoy had left Dover and was travelling to protest outside the French embassy in London.