Manama: Portugal has confirmed that it is taking no risks when the world’s most influential leaders gather in Lisbon for the three-day North Atlantic Treaty Organisation (Nato) summit.

The country's exceptionally draconian measures aim to help guarantee "internal security and the maintenance of public order" during the November 19-20 meeting of the alliance, the Ministry of Internal Affairs said, quoted by the Lusa News Agency.

Concerned over the threat that terrorists, anarchists and extremists pose, Portugal has mobilised all its law enforcement agencies and the Ministry for Internal Affairs has issued strict orders that no police officer be given leave for the duration of the summit.

The government does not want that the presence of world leaders such as US President Barack Obama could result in huge demonstrations and major disruption in the Portuguese capital, the agency said.

Immigration officers have reportedly been told they are to return to monitoring the border with Spain, while their numbers will be boosted at airports across the country.

Officers from other police forces have been placed on standby to meet any potential need for reinforcements during the summit and to confront extremist groups.

The civil aviation authority said that it would reduce the rate of flight arrivals by half, from 23 per hour to 14 from noon on November 18 until noon on November 21.

Rui Pereira, the interior minister, said Portugal’s terrorist threat alert level would be high during the summit, but insisted there was “no reason for alarm."

"There is no reason for alarm although we must be vigilant and pay attention to every sign”, Pereira said. "Caution is necessary regarding commercial aviation, which has been terrorist groups’ preferred medium," he said.

The Portuguese government will also impose border controls from November 16 to November 20 November, effectively temporarily suspending Schengen agreement rules.

The Schengen Agreement, which Portugal signed in 1995, encourages the free movement of people between signatory states. However, under the accord, the temporary re-imposition of border controls under exceptional circumstances is permitted.

The government attributed the "exceptional measure to suspend the unchecked movement of people" to the Nato summit’s "dimension, media visibility and complexity," including the presence of "high level officials from 28 nations."

Portugal’s postal service operator CTT has improved its mail and package verification security operations ahead of the summit, sources cited by Portugal News reported.

Pedestrians and drivers in Lisbon will on Monday begin to feel the effects of the restrictions imposed throughout the Park of Nations area for the upcoming NATO summit. The restrictions will extend until November 21, The Portugal News reported.

In a letter sent to the Association of Residents and Business owners in the Park of Nations, the Lisbon Metropolitan Unit, Cometlis, said that no people or vehicles would be allowed between midnight on November 15 and midnight on November 21 within the perimeter of the restricted area unless they were associated with the summit or had the required credentials to that effect. Only authorised vehicles and people will be allowed passage, but will be searched.

A police spokesperson advised residents and employees in the Park of Nations area to use public transport next week.

"It is also recommended that between November 17 and 20, businesses avoid meeting clients or suppliers in and around the Park of Nations due to significant delays that could occur because of the restrictions," the police said.