Croatia has allowed thousands of migrants stranded on the border with Serbia to cross into its territory, clearing a bottleneck that had been building up in the Balkans.
More than 3,000 migrants were stranded in cold and wet weather at the Berkasovo border crossing before the gates were opened on October 19.
Melita Sunjic, a spokeswoman for the United Nations refugee agency (UNHCR), said that "without any announcement, the borders opened. When the borders opened, everybody rushed" over.
Many remain stranded on the Croatian side of the Croatia-Slovenia border, however.
Tension had been building among migrants after their route to Western Europe was blocked by new border restrictions.
Citing security concerns, Hungary has closed its borders with both Serbia and Croatia, forcing migrants travelling north to travel via Croatia and Slovenia.
Croatia has asked fellow European Union-member Slovenia to accept 5,000 migrants daily, but its northern neighbor said it would only take half that number, citing stricter border controls by neighboring Austria.
The moves have led to a build-up of migrants and refugees on Croatia's border with Serbia.
Czech volunteers working with the organization Refugee Help say more than 3,000 refugees, including small children, are waiting in the cold and rain at the Berkasovo border crossing on October 19.
Describing the situation there as "out of control," they say three or four buses packed with migrants arrive at Berkasovo every hour from Macedonia, while only one bus per hour is leaving.
Jan Pinos, one of the volunteer coordinators, warns of increasing tension among desperate refugees.
"We warned Serbian authorities not to take more refugees here," he tells RFE/RL.
"We need police, doctors and organizational support on [the] spot, otherwise we will have to witness a tragedy," Zuzana Lenhartova, another volunteer coordinator working with Refugee Help, said in a written statement.
Earlier on October 19, before Croatia opened its border, the United Nations refugee agency (UNHCR) said Serbia was hosting more than 10,000 migrants on October 19, with more on the way.
"It's like a big river of people, and if you stop the flow, you will have floods somewhere," UNHCR spokesman Melita Sunjic said from the Serbia-Croatia border. "That's what's happening now."
More than 600,000 people fleeing violence and poverty in countries such as Afghanistan, Iraq, and Syria have reached Europe so far this year.
Most migrants are Syrians who travel from Turkey, via Greece, Macedonia, and Serbia, aiming to reach Austria and Germany.
Slovenia, a country of 2 million people, has found itself in the path of Europe's biggest mass migration since World War II after Hungary sealed its borders with Croatia with a fence on October 16.
Budapest had previously sealed the border with Serbia, saying the mainly Muslim migrants posed a threat to Europe's prosperity, security, and "Christian values."
In Croatia, a train carrying 1,800 people was blocked by Slovenia from entering its territory early on October 19, with police allowing only 500 of the "most vulnerable" -- mostly women and children -- to cross.
Slovenian Interior Minister Vesna Gyorkos Znidar accused Croatia of sending a "very big number of immigrants, out of a previously arranged framework," adding that it is "absolutely unacceptable."
She said Slovenia can manage up to 2,500 people a day and "can't accept an unlimited number" of migrants.
"Slovenia first said it could receive up to 8,000 migrants [daily], then 5,000, then 2,500, and now it has been reduced to zero," Znidar's Croatian counterpart, Ranko Ostojic, replied. "It would mean that the whole burden is being left to Croatia."
Explaining Slovenia's limitation on October 18, Interior Ministry spokesman Bostjan Sefic said Austria had limited its own intake to a maximum of 1,500 people a day.
Austria denied that, with Interior Ministry spokesman Hermann Muhr saying the "status quo" would continue. He and other officials didn't specify numbers.
The European Union has agreed to a plan to share out 120,000 refugees among its members, but several member states have resisted. The bloc is also courting Turkey to try to convince it to stem the flow of migrants across its territory.
With reporting by Reuters, AP, and the BBC