Croatia has opened its border with Serbia, removing one of the bottlenecks for thousands of stranded migrants trying to make their way north.
About 3,000 people had been stuck in cold and wet weather in the Serbian border village of Berkasovo, after Croatia moved to curb new arrivals.
The UN refugee agency (UNHCR) had described conditions there as "dire".
Many remain stranded near the Croatia-Slovenia border. The Balkan route has been squeezed by restrictions.
Hundreds of thousands of migrants, many from Syria, Africa and Afghanistan, have been making their way from Turkey to the Balkans in recent months, in a bid to reach Germany, Sweden and other EU states.
UNHCR spokeswoman Melita Sunjic, who is at the Berkasovo crossing, said late on Monday: "Without any announcement, the borders opened. Everybody rushed."
Describing the conditions at the camp there earlier, she had said: "We have a lot of small children. We have disabled people, we have people who got sick on the road.
"This is not a place for people, they can't sleep - they can just stand upright in the mud."