Portugal has formally reimposed border controls with other European countries to improve security before the Euro 2004 football championships in June.
Portugal's police have new equipment for the games
The restrictions, which were announced several weeks ago, come into force on Wednesday and will last until July.
Hundreds of thousands of foreign football supporters are expected to arrive in Portugal for the event.
The border controls are among measures to protect against anything from hooliganism to terror attacks.
Other measures include the use of Nato advanced warning aircraft, Awacs.
The border controls have been reimposed well in advance of the tournament, which starts on 12 June, and before the six-day Rock in Rio music festival being staged in Lisbon from Friday.
Normally there are no border checks between most EU member states and several other European countries.
With controls in place, EU citizens arriving in Portugal must show their passport or identity cards. Non-EU citizens must have their passport and a visa, if appropriate.
The BBC's Alison Roberts, in Lisbon, says mobile patrols are being put in place on roads from Spain rather than reopening disused border posts.
The border controls in Portugal will stay in place until 4 July, but their reimposition coincides with the second in a series of go-slow strikes by immigration officials.
To minimise their impact, the government has threatened to activate legal mechanisms obliging officials to turn up for work.
Some countries taking part in the championships are taking their own measures to help curb violence at the tournament.
The UK Home Office is planning its own security operation to prevent any violence by England fans. It will include the biggest-ever surveillance of ports and airports, to try to ensure known or suspected hooligans do not leave the country.
UK forces and their Portuguese counterparts say they will stick to the plan of "softly, softly policing" unless fans' behaviour calls for more forceful tactics.
If there is any violence, the England team could be thrown out of the tournament.
Euro 2000 was marred by violence from England fans when it was held in Belgium and Holland.