Some 300 migrants, the largest group in many months to try to march across Serbia into Hungary, were nearing the border on Sunday, the UN refugee agency said.
The group of mostly young men from Afghanistan and Pakistan had set off from Serbia's capital on Friday to protest against the closure of Hungary's border as well as tough new measures introduced this month to tighten border security.
The group reached the town of Subotica near the border on Sunday morning, continuing their march towards the frontier in early afternoon in the hope of gaining entry into the European Union.
Serbia lies on the so-called Balkan route taken by hundreds of thousands of migrants from the Middle East, Asia and Africa since last year on their way to western Europe.
Although the route was effectively shut down in March, migrants have continued to cross the region in smaller numbers, often with the help of smugglers.
The migrants began the 200 kilometre (120 mile) trek north escorted by a police car and carrying blankets and backpacks, many bearing the distinctive blue logo of the UN refugee agency (UNHCR).
Abdul Malik from Afghanistan had said the group was "going to Hungarian border" and was "gonna change something by walking and not eating."
Hungarian media quoted one of the marchers as saying he planned to stage a hunger strike at the border.
The number of migrants blocked inside Serbia has grown significantly since Hungary introduced tough new measures this month to stop them crossing the border.
According to UNHCR in Serbia, up to 2,800 migrants were in the country on Friday, mostly in makeshift along the Hungarian border.
Last weekend Belgrade decided to launch joint police and army patrols to beef up its borders.
Serbian authorities said recently that 102,000 migrants had been registered since the start of the year -- more than 500 a day.