(Adds Rio government comment, context)

By Raymond Colitt

BRASILIA, Nov 14 (Reuters) - Police frequently execute criminal suspects and participate in organized crime in Brazil's violence-plagued cities, a United Nations special envoy said on Wednesday.

The findings by Philip Alston, the U.N. special rapporteur on extrajudicial executions, reinforce human rights concerns that police increasingly use brutality to placate popular discontent with rampant crime in Brazil, which has one of the world's highest homicide rates.

"The stories they told were tragic," Alston said of the interviews he had with witnesses and victims' relatives.

There was strong evidence that many of the 694 people killed by police in the first half of the year in Rio de Janeiro alone were extrajudicial executions, Alston told a news conference in Brasilia at the end of an 11-day visit.

Repeated waves of violence, which have shut down entire neighborhoods in Rio de Janeiro and Sao Paulo over the past year, increased pressure on President Luiz Inacio Lula da Silva and state governments to get tough on crime.

Some off duty police form militias or death squads that kill to protect racketeering schemes or prevent gangs from undermining their control, the U.N. rapporteur said.

Last week the members of two such groups accused of having killed hundreds of people were arrested in northeastern Brazil.

"The people of Brazil did not struggle valiantly against 20 years of dictatorship (1964-1985) ... only in order to make Brazil free for police officers to kill with impunity in the name of security," Alston said.

He noted a police anti-drug operation in Rio de Janeiro in June that killed at least 19 people.

"I had to conclude that this operation was conducted for political reasons. It doesn't enhance security, it may enhance opinion polls in the short-term," he said.

The U.N. envoy said he was "staggered" to hear that the operation involving 1,350 officers killed 19 people because of "resistance" but seized only a dozen weapons and did not arrest or kill any of the major drug dealers.

The Rio de Janeiro state government said in response that it would continue to act against major drug traffickers.

"The conflicts are undesirable but in the name of civil and human rights, there is no way to back down from this obligation," it said in a statement.

Lula launched a $3.3 billion plan in August to curb crime with tougher police enforcement and more social welfare policies.

Alston, who visited at the invitation of the Brazilian government, said he would recommend additional measures in his final report in March.

His preliminary recommendations included higher police pay, better forensics, improved witness protection programs and effective investigations into police killings.

(Editing by Eric Beech)

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