LONDON (AFP) - Children were separated from their families by police in chaotic scenes as migrants broke through Serbia's border with Hungary on Wednesday, Amnesty International said.

The rights group said that at least nine people, including at least four children, were separated by Hungarian police after migrants surged forward and broke through a border fence and the police responded with pepper spray, causing panic.

"The families are desperate to be reunited with their children. Not only have they experienced the traumatic journey to the border and the use of force by the police - they have now lost the security of being with their parents," said crisis response director Tirana Hassan in a statement from the scene.

The rights group said the children were thought to have been brought to a nearby border control building.

"The Hungarian authorities must immediately hand these children over to their families," Hassan said.

The violence broke out after hundreds of people became frustrated at Hungary's closure of its border, previously a major crossing-point into Europe for migrants and refugees fleeing war in Syria and elsewhere and a flashpoint in Europe's refugee crisis.

The situation escalated and there were clashes between riot police and migrants, as police fired tear gas and water cannon and migrants threw sticks, stones and bottles.

Some migrants managed to pull down wire mesh and run into Hungary.

A father of an eight-year-old boy told Amnesty: "My child was taken from me as I was holding his hand and we've been separated ever since."

Amnesty said that another man was searching for two children aged six and eight, and that eyewitnesses had seen Hungarian police pick up a mother and child and take them away.