Croatia opened its border with Serbia on Monday to let in thousands of refugees who had been stuck in the country nearly two days after Hungarian border was shut.
Nearly 3,000 refugees had been stranded on the border in the mud and rain until the gates were opened.
UNHCR spokeswoman Melita Sunjic said that "without any announcement, the border opened. When the border 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.
Rough weather created more desperation for the refugees. Ankle-deep mud meant they could not move easily while rain water filled their tents and soaked their blankets.
Matija Posavec, the governor of Medjimurje - Croatia's northernmost county which borders Slovenia - said it was impossible to halt the refugees who intend to reach wealthier nations in Europe, such as Germany.
"The Republic of Croatia has asked these refugees to stay at our reception centres until their status is resolved, but they've all refused it," Posavec said.
"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."
More than 2,100 refugees also walked into Slovenia from Croatia after the machinist of the train which carried them refused entry into the country.
Slovenia announced on Monday that it had eased restrictions against refugees and allowed 5,000 of them to enter the country, despite its previous decision to take in only 2,500 refugees per day.
On the other hand, the country is trying to keep its border under control to prevent possible further influxes.
The Slovenian government proposed a law overnight to send Slovenian troops to the country's borders in addition to police units as thousands of refugees flooded the into country.
It is expected that the parliament will pass the bill later on Tuesday.
Prime Minister Miro Cerar told the national Radio Slovenia that "this is not about enforcing an extraordinary condition, it is about strengthening control on the border."
Following the massive influx across the border, Slovenia on Tuesday called on Europe to show solidarity in handling the growing humanitarian crisis on the continent.
"Slovenia calls on the European Union states and institutions to engage actively in dealing with this disproportionate weight for our state,” the government said in a statement.
“European solidarity is being challenged. It is delusional to expect a country of two million to [accomplish] what much larger countries haven't been able to," it added.