Hungarian police temporarily closed a Serbian border crossing on Thursday after a large migrant group demanded to be let in.
A week earlier, as many as 70 people tried to enter Hungary at another Serbian border crossing under cover of night, prompting security staff to fire warning shots in the air.
Hungarian police have reported a sharp rise in attempts to cross the country's southern borders since December.
"The Tompa road border crossing has been temporarily closed as undisturbed crossing cannot be guaranteed due to a large group of migrants near the Serbian side," Hungarian police said.
A Serbian border police officer told AFP that about 100 migrants were on a strip of land between Serbia and Hungary, demanding that Hungarian authorities allow them in.
He said they were peaceful but would not say how they had reached the border area.
"We are peacefully protesting," one man told the HVG news website. "We want to be allowed to continue our journey towards western Europe."
Some in the group said they were from Syria, Afghanistan and Yemen, and that they would stay in the area between the two borders until they were let through.
Placards held by some migrants read "We are just refugees, not criminals", and "Let us cross".
Later on Thursday, Serbian public broadcaster RTS showed images of "a few hundred" migrants, including women and children, sitting or lying in sleeping bags.
Serbia's Defence Minister, Aleksandar Vulin, was on his way to the border.
An adviser to nationalist Hungarian Prime Minister Viktor Orban said last week that more than 3,400 attempts were made last month to cross the country's southern borders, compared with several hundred a month last year.