Macedonia gave up on its attempts to close its borders to curb the influx of people seeking entry.
It's the latest dramatic chapter in Europe's increasing refugee crisis.
Thousands of migrants had been stuck for three days in an area between Greece and Macedonia described as 'no man's land'.
It came after the Former Yugoslav Republic of Macedonia declared a state of emergency and closed its border with Greece.
Riot police used tear gas and stun grenades to drive back crowds, but they were overwhelmed by the thousands of people who tore through police lines or ran through nearby empty fields.
In the end, authorities gave in and allowed the migrants access and facilitated trains and buses to transport them north to Serbia.
Melita Sunjic, from the United Nations Refugee Agency, has described the state in which many are arriving.
"We see more and more families with children. We see them taking their old parents with them in wheelchairs, these people are coming mostly from refugee producing countries like Syria."
Serbian Defence Minister Bratislav Gasic has visited a reception centre housing some of the migrants.
He says authorities are working hard to issue documentation to allow them to pass through Serbia and continue on their journey.
"Here we have over 4,000 people now, we're all very well organised. All state institutions, we are very proud. Serbia is showing once again that in a time of crisis, we are all united. We have hospital staff from Preservo present here, Red Cross, gendarmerie, police and army all in one place and with one task only - to give these people human treatment. To give them food and aid so that they can continue their journey."
Authorities say last month alone, about 50,000 migrants arrived on Greek shores by boat from Turkey.
They say about 2,000 are now arriving at the border into Macedonia every day.
Macedonia is a transit point to western Europe via Serbia, from where they continue on foot to Hungary to gain access to European Union countries.
French Foreign Minister Laurent Fabius says the growing crisis needs to be dealt with by all countries.
"What does solidarity mean? It means that each should take their part of the issue in particular concerning the political 47asylum and it's very hard to imagine that a country can deal with it all alone. Solidarity also means that obviously there are actions to be taken in relation to the countries where those migrants are coming from. At the same time we have to be firm with people smugglers as well as firm in saying that we possibly can't accept all the economic migrants. On the other side it is necessary to distinguish between people who are entering Europe by legitimate reasons and those who are coming just for economic reasons."
Despite the uncertainty and anguish greeting many migrants in Europe, hundreds continue to make the perilous boat journey.
The Italian coastguard says it coordinated the rescue of 4,400 migrants in the Mediterranean after receiving distress calls from more than 20 overcrowded vessels drifting in waters off Libya.
It says it has been one of the biggest single-day rescue operations ever and so far there appear to be no reports of casualties.
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