Hundreds of people pass through border from Greece in darkness after failed bid by Macedonian police to stop them.
Thousands of exhausted refugees from the Middle East, Asia and Africa have crossed on foot from Macedonia into Serbia on their way to the European Union.
The rush over the border on Monday came after Macedonia effectively lifted the blockade of its border with Greece on Sunday, after thousands of refugees stormed past Macedonian police who had been trying to stop their entry by force.
Continuing their journey, about 7,000 refugees, including many women with babies and small children mostly from Syria, had crossed into Serbia over the weekend by Monday morning.
Some were pushed in wheelchairs and wheel barrows or walked on crutches.
Meanwhile, hundreds more entered Macedonia from Greece on Monday.
The number of refugees now crossing into the Western Balkans is worrying many EU politicians and left the Balkan countries struggling to cope with the humanitarian crisis.
After entering Serbia, the refugees, often fleeing wars and poverty, head towards EU-member Hungary from where they often continue further north to richer EU countries, such as Germany and Sweden.
“I was at a camp close to the Macedonian border. I have not seen any barb wires, fence or tensions between about 100 border police and several hundred refugees,” Al Jazeera’s Aljosa Milenkovic said, reporting from the Macedonian-Serbian border.
“They are gathered there and then taken on foot to the UN refugee agency buses. Some of them are happy that they are over the Macedonian border.”
“I am from Iraq, I want to go to Germany,” Ali told the AP news agency, barely speaking with exhaustion as he sat on a dusty field with columns of refugees heading for an overcrowded asylum centre in the Serbian border town of Presevo.
After they formally ask for an asylum, arrivals have three days to reach the border with Hungary which is rushing to build a barbed wire fence on its border with Serbia to block the refugees.