Röszke (Hungary) (AFP) - Hundreds of migrants broke through police lines in Hungary near the Serbian border Wednesday for a third day running, as the army began mobilising for a possible role in guarding the frontier.

The break-out involving at least 400 migrants took place near the flashpoint town of Roszke, where migrants have to wait at a collection point before being taken to a nearby centre for registration.

Shouting "No camp!" they scattered in all directions, some heading for a nearby motorway leading to Budapest, which police then temporarily closed down.

The migrants later agreed to be taken to a refugee camp in western Hungary.

It was the latest in a series of tense confrontations between police and desperate migrants and refugees as the Hungarian authorities struggle to cope with thousands of new arrivals every day.

In the 24 hours to midnight (2200 GMT), 2,770 people were intercepted nationwide, police said.

At Roszke, the main crossing point from Serbia, there were 2,529, including 455 children. Most were Syrian, Afghan or Pakistani.

On Tuesday, there was a series of similar breakouts in the same area involving several hundred people, with police using pepper spray on one occasion to move a group off a main road.

Hungary's southern border has become a major entry point into the European Union for migrants and refugees fleeing war and misery in the Middle East, Africa, and Asia.

More than 165,000 migrants have crossed into Hungary so far this year. Most seek to travel on to Germany via Austria.

The army on Wednesday launched an exercise called "Decisive Action" to prepare for a possible role in border control, pending a vote in parliament later September on the issue.

"It is our job to make sure Hungary is defended," General Tibor Benko told a news programme on the M1 channel.

Hungary recently completed a razor-wire barrier along its 175-kilometre (110-mile) frontier with Serbia, but it has failed to stop large numbers of people getting through.

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It is currently building an additional four-metre (13-foot) fence despite widespread criticism from western European leader.

Prime Minister Viktor Orban vowed earlier this week to speed up the new barrier's construction.

"Everyone has to get ready to do intensive work in the coming weeks," Orban said in a newspaper interview.

An army source told AFP that troops had begun preparing Wednesday for work on the fence.

A senior member of Orban's ruling Fidesz party on Tuesday gave the deadline for the fence's completion as "end-October, early-November".