Croatia has opened its border with Serbia, letting in thousands of refugees who had been stranded for nearly two days in the cold.

The opening on Monday partially cleared the human bottleneck that has been building up in the West Balkans.

The humanitarian crisis has been growing since Saturday, when Hungary closed its border with Croatia, creating a backlog of refugees in the region.

Hungary closes its border with Croatia

UNHCR spokeswoman Melita Sunjic, on the Serbia-Croatia border, said that “without any announcement, the borders opened. When the borders opened, everybody rushed” over.

“The last person to go was a young boy without a leg, and we helped him cross in a wheelchair,” Sunjic said.

There were between 2,000 and 3,000 refugees stuck on the border in the mud and rain when the gates opened.

An empty field littered with some of the refugees’ belongings showed that they were desperate to get on the move again. Stuffed toys, a milk bottle, a child’s rubber boot, crayons scattered in the mud and soaked blankets were among the items left behind.

The road leading from Serbia into Croatia testified to the treacherous conditions the refugees have endured since Croatian police stalled the flow. Ankle-deep mud was everywhere, and there were pools of water inside the tents next to blankets where refugees were sitting hoping for some cover.

Cleaners went in after the refugees left, preparing the site for the next wave of asylum seekers expected to arrive soon.

Al Jazeera’s Ivan Pavkovic in Opatovac, Croatia, spend the day at a refugee camp near the border with Serbia.

“The situation could become more intense and authorities are now trying to strengthen their control around this camp. We saw dozens of new police officers, there is more reinforcement,” he said.

“I spoke with many of the refugees. They are not very aware of what’s happening between all of these countries and their issues over whether or not to accept them.

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“It seems these people can expect more obstacles on their way to Europe but they are determined to go. They mainly mention Germany, Denmark, Norway or Sweden as their final destination, and they hope to find a better life there.”

The UNHCR estimates there are more than 10,000 refugees in Serbia – more than double the daily average in the past month.

Croatian government leaders argue that it is pointless, if not impossible, to stop people who overwhelmingly express determination to reach wealthier, more liberal nations in Europe, chiefly Germany.

“The Republic of Croatia has asked these refugees to stay at our reception centres until their status is resolved, but they all refuse it,” said Matija Posavec, governor of Medjimurje, Croatia’s northernmost county bordering Slovenia.

“They could have stayed on board the train. They could have stayed at the reception centres, but none of them really wants that. … They just want to pass.”

A large group of some 2,100 migrants walked into Slovenia from Croatia on Monday, after a train carrying them through Croatia to the Slovenian border was denied entry into Slovenia.

Slovenia eventually admitted Monday’s new border arrivals, leaving just 500 from the overnight train still waiting in the rain by the afternoon.