Military guard for Spanish summit

Warships in Barcelona port are part of the extensive security arrangements

BARCELONA, Spain (CNN) -- Security is tight in Barcelona ahead of a European summit starting on Friday, with police, army and navy on alert for violence.

While a peaceful trade unionist protest of about 100,000 caused no problems in the city on Thursday, the 8,000 police on summit duty are prepared for violence from other groups.

Police are on alert for any terrorist threat or mass demonstration.

"I am very confident everything will go well," says Julia Garcia Valdecasas, one of the government officials playing a key role in protecting leaders of the 28 European nations due in Barcelona.

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"Barcelona will be able to host the European summit and also the protesters who oppose its policies."

Spanish media reported extensive security arrangements were in place, including F-18 fighters on alert to intercept intruding aircraft and a NATO airborne warning and control system plane patrolling the skies, Reuters news agency said.

Ground-to-air missiles have been brought in to the military section of Barcelona airport and naval patrol ships are watching the Catalan coast, the Spanish newspaper El Mundo said.

Organisers say they don't want any violence this time but concede they can't be sure it won't happen.

"You will see how the politicians are going to meet locked inside their own castle," says Marcelo Exposito of the Movement for Global Resistance.

Police inspect the sewer tunnels below the Barcelona hotel where officials are to stay while attending the summit

"And what we usually say is, 'Today the city is blocked -- actually it's an act of violence against people -- and we're going to unblock the city."

But not if the police have their way. They have been training for months in riot control. It's the tightest security Barcelona has seen since the Olympics a decade ago.

While police have sealed off the conference site with barricades, officials admit that violence across town could jeopardize the summit.

Officials consider ETA the biggest threat. Police have arrested 50 suspected ETA members or collaborators this year, putting a dent in the group's plans to attack the summit.

Authorities also have not ruled out Islamic terrorist attacks.

"The less said about security the better. We're not going to reveal our exact plans," says Valdecasas.