Hundreds of migrants from the Middle East and Asia are marching through Belgrade on their way to Hungary and are planning a hunger strike to underline their demand for passage to western Europe.
Last month Hungary, Serbia's northern neighbour and an European Union member, allowed police to send back illegal migrants detained within eight kilometres of its razor wire-fenced frontier with Serbia.
It has also limited the number of daily entries to a maximum of 30, creating a bottleneck in Serbia.
A group of around 300 men, mainly from Afghanistan, Pakistan and Iran, marched across the main bridge between Belgrade's city centre and the Novi Beograd area on Friday, with a police car leading the column.
There were no women or children in the group.
A police officer who declined to be named said the migrants had started a march to the Hungarian border, around 200 kilometres to the north.
"We are here to guarantee their safety and prevent accidents en route," he said.
Earlier in the day, the group also announced a hunger strike, said Vladimir Sjekloca, a manager of Belgrade's Asylum Info Centre.
To cope with the inflow of over 100,000 migrants that passed through Serbia so far this year, the country's police and army last week formed joint border patrols.
More than 650,000 people passed through Serbia last year on their way to the EU.
On Friday, Aleksandar Vulin, Serbia's minister for social affairs, said as many as 3000 migrants were stranded in the country.
"It will not suffer because EU countries cannot find a joint solution."
Unlike in 2015, when countries tried to organise the orderly passage of migrants, the closure of the Balkan route prompted many to cross borders illegally or through human trafficking networks.