Ecuador has brought in new rules to stop Venezuelan migrants entering the country without a passport, leaving many stranded in neighbouring Colombia.
Thousands of Venezuelans fleeing their country's economic and political crisis have been crossing into Ecuador from Colombia using only identity cards.
Most are heading south to join family members in Peru and Chile.
Colombia has protested against the move, saying vulnerable people will be trapped on its side of the border.
In a separate incident, residents of a Brazilian town attacked a Venezuelan migrant camp on Saturday and drove the occupants back across the border.
Venezuela has suffered for years from high inflation and the chronic shortage of food and medicines.
More than a million Venezuelan migrants have entered Colombia in the past 15 months, according to official estimates, and more than 4,000 have been arriving at Ecuador's border every day.
Ecuador's decision to shut the border for those without passports went into effect on Saturday.
Many migrants, who have been walking or hitching rides for weeks and are exhausted by the time they reach the frontier, only carry ID cards.
Early on Saturday, about 300 Venezuelans were lined up at the Rumichaca border crossing outside the Colombian city of Ipiales, and many said they had no passports to gain entry to Ecuador.
Gabriel Malavolta, a 50-year-old mechanic, left Venezuela three days ago aiming to make the overland route to Lima, Peru, through Ecuador.
He has a passport but his fiancee, Yenny, had only an ID card.
"I don't know what we're going to do, but we can't go back. I'm not sending my fiancee to go back and go hungry," he told Reuters news agency at a Red Cross tent.
"You've no idea what it's like [in Venezuela]. Whole families eat from the trash."
Another migrant, Regulo Guaita, said: "That was truly a surprise. We found out today. It's very sad because there are many, many Venezuelans leaving and (with this measure) they won't let them leave. I don't know what they'll do now."