By Brad Brooks and Rodrigo Viga Gaier

RIO DE JANEIRO (Reuters) - Portugal's education minister was robbed at knifepoint on Saturday at the Olympic lake where rowing events are taking place, but escaped unharmed, authorities said.

Education Minister Tiago Brandao Rodrigues and an aide were walking near the event area when they were assaulted.

The assailant was quickly located by police at the lake, a few blocks away from Ipanema beach, and arrested. Rodrigues' belongings were returned to him.

It was the latest incident in a series of robberies that have hit visitors since the Olympics opened on Friday. Rio has dramatically lowered its crime levels in recent years, but there has been a spike in murders and assaults in recent months.

Authorities said that 85,000 soldiers and police patrolling streets for the Games - double the number of security officials in place during the London Games in 2012 - would keep Rio safe.

But even top security officials are not immune to attacks.

Brazil's Ministry of Justice confirmed that Felipe Seixas, coordinator of security for special events within the government's Secretariat for the Security of Large Events, was the target of an assault as he left the opening ceremony Friday.

Seixas, who was accompanied by two undercover police, was confronted by would-be robbers outside Maracana stadium.

One of the estimated five robbers, who were armed with knives and riding bicycles, was shot and killed by one of the undercover officers accompanying Seixas.

Meanwhile, authorities are still trying to determine how a rifle round ripped through the media tent at the venue hosting equestrian events and landed on the floor near a journalist.

It is not clear if the round came from the weapon of a soldier or was fired from a nearby slum.

Both security personnel and drug gangs in Rio are armed with the same high-caliber weapons that could have fired the projectile.

(Editing by Bill Rigby)