Portuguese authorities have put in place heavy security ahead of the NATO summit to prevent any outbreak of violence.

Logos of the Lisbon NATO summit are seen at the press center in Lisbon, capital of Portugal, on Nov. 18, 2010. The NATO summit will be held from Nov. 19 to 20 in Lisbon. [Wu Wei/Xinhua]

As leaders from NATO's 28 member states are meeting on November 19 and 20 in Lisbon, Portuguese authorities have decided to beef up security in order to avoid a repeat of the violent riots that broke out at last year's summit in Strasbourg, France.

The country's borders have been patrolled by police and helicopters since the beginning of the week in an attempt to turn back extremist demonstrators and troublemakers.

Around 7,000 security officers will be deployed between the venue and the city center, where several anti-Nato demonstrations are expected to take place.