The biggest security operation in Portugal’s history is underway, as the country has tightened security measures to host a NATO summit in the capital, Lisbon, Portuguese police say.

The security operation which involves the Portuguese Navy and Air Force is expected to bring more than 5,000 people to the capital, euronews reported on Wednesday.

The two-day summit will kick off on Friday.

The NATO leaders are expected to sketch plans for a progressive withdrawal of troops from Afghanistan.

Strict traffic restrictions will also be in place during the summit and public sector workers will be sent home early for the weekend.

Portugal has also suspended the Schengen Treaty on free travel within a 25-nation area, from Tuesday until the end of the summit as part of its efforts to prevent violent anti-NATO and anti-globalization protests.

According to officials, border controls were being reinstated before and during the summit in order to keep out “the usual troublemakers.”

At least 50 people have been denied entry into Portugal.

“The people who have been refused entry may have been trying to come to Portugal with the purpose of disturbing order and public security,” Border and Immigration Service chief Manuel Palos said.

However, he did not give the nationalities of those arrested and detained.

Officials are trying to prevent a repeat of the violence which marred last year’s summit in the French city of Strasbourg.

Several groups are set to hold anti-NATO rallies to protest the military alliance’s operations in Afghanistan.