Thousands of extra security agents for World Cup 2014

Around 30,000 men of the Brazilian Navy took part in military exercises on Thursday

Brazil has announced a security boost of around 70,000 officers to police this year's football World Cup.

In total, 170,000 people are expected to be deployed to guarantee security across the 12 host cities.

On Wednesday, President Dilma Rousseff said the armed forces could also be brought in if needed.

Since last June, protesters have staged a number of street marches, with some violent episodes, raising concerns about security at the tournament.

Earlier this month, a cameraman died after being hit by fireworks apparently launched by protesters.

In January, the organisers had suggested the tournament's security personnel would total 100,000 people.

'Potential violence'

But at a security workshop on Thursday, the organising committee said this figure should be increased.

"We have a lot of concerns, not so much about protests which are a democratic right, but rather about potential violence. We are committed to preventing violence during any protest activity," a Justice ministry official, Andrei Rodrigues, told reporters.

Around 150,000 personnel will be from the armed services and police.

But an extra 20,000 security stewards will be trained to work inside the 12 stadiums of the tournament, organisers say.

Also on Thursday, Brazil launched its first naval preparatory exercise ahead of the World Cup.

Among the operations exercised was the raid of a "hostile" ship in the waters off Rio de Janeiro

Around 30,000 men, in more than 60 ships and 15 planes took part in the operations.

World football governing body Fifa's security director, Ralf Mutschke, said he was confident Brazil was going to host a safe tournament.

"The teams have great faith in the level of security going to be provided," Mr Mutschke said at the close of a security workshop for officials of the 32 teams which will be competing in Brazil.

Brazil saw a big wave of protests in June, as the country hosted football's Confederations Cup.

What started as a protest against an increase in bus and underground fares grew into a much larger movement.

More than a million people took to the streets against corruption and excessive spending in preparations for the World Cup and the 2016 Olympics, which Rio will host.