SAO PAULO (REUTERS) - Former Brazil soccer star Ronaldo said on Thursday that authorities must crack down if violent protests break out during the World Cup and urged police to "bring down the clubs" on vandals.

Strikes and small demonstrations are widespread two weeks from the June 12 opening match as many Brazilians who see no benefits from the costly event use the tournament as an opportunity to air a growing range of grievances.

"Protests are always valid ... But the moment masked vandals appear the police have to contain them," Ronaldo said at an event sponsored by the newspaper Folha de S. Paulo. "I think they have to bring down the clubs, get them off the street."

Security is one of the government's primary concerns. Fernando Grella, the top security official for Sao Paulo state, told Reuters that police are assembling possible criminal cases against a small number of protest leaders, perhaps a few dozen, who he said are conspiring to "commit violent acts, break things, vandalise and attack people."

Ronaldo's warning came days after he said he was embarrassed for his country's failure to complete the promised infrastructure in an interview with Reuters that triggered a sharp response from President Dilma Rousseff.

"As I told Reuters, my embarrassment is on behalf of the population that really expected these large investments ... airport reforms, urban mobility, everything that was promised and not delivered," he said.

Ronaldo is a member of the Local Organising Committee and one of the most visible backers of soccer's world governing body FIFA and the Brazilian government.

He was a member of the Brazil teams that won the World Cup in 1994 and 2002.