Prime Minister Jose Manuel Durao Barroso made the decision to increase security following an emergency meeting held late on Sunday with the heads of Portugal's police and intelligence forces in Lisbon.

"While there is no specific threat directed against Portugal at this moment, the decision was made to reinforce preventive measures," a government source told the Lusa news agency.

The interior ministry said it had placed airports and embassies considered "sensitive" under tight security and had boosted its intelligence gathering.

Immediately after the attacks in Madrid on Thursday which killed more than 200 people, Portugal strengthened its border controls with Spain, which is its largest trading partner.

Portugal, like Spain, was a vocal supporter of the US-led war against Iraq and the bombings in Madrid have heightened fears that the country could also be a target of a terrorist attack, especially when it hosts the 16-nation Euro 2004 championship.

"The lesson of Thursday is that Portugal needs to reinforce all of its preventive measures during the upcoming events," President Jorge Sampaio said in an interview published Monday in daily newspaper Comercio do Porto.

The Portuguese air force is to close the airspace over stadiums, and patrol the skies with F-16 fighter jets on match days, according to an air force source cited by weekly newspaper Expresso over the weekend.

The newspaper said the measure was asked for by organisers who feared hijacked planes could endanger spectators' safety during the event.

Portugal is expecting an extra 500,000 visitors for the tournament which will take place in eight cities from June 12 to July 4. The country usually receives 1.4 million tourists in June.

Interior Minister Antonio Figueiredo Lopes said on Friday Portugal would most likely reimpose its border controls with Spain and other European nations during the championship, the largest sports event worldwide after the Olympics and the football World Cup.

Portugal has signed the Schengen agreement, which abolished border checks between 15 European countries, but Lisbon can use a clause in the treaty which allows them to temporarily be reimposed under special circumstances.

Until the attacks in Madrid the minister had said Portugal was only studying the possibility of reimposing the border controls.

Concern over the risk of terrorist attacks increased on Monday after top-selling daily newspaper Correio da Manha reported it had received two telephone calls from men claiming to be members of the al-Qaeda extremist group who said Portugal would be struck before the European football finals begin.

The paper said one of the callers, who spoke in Spanish, said at least two of the men who were responsible for the train bombings in Madrid were already in Portugal.

The second caller, who spoke in Portuguese with a foreign accent, said he was an Algerian who had lived in Portugal for ten years.

The paper said it immediately reported the calls, which it received on Friday, to the interior ministry which is investigating their credibility.