One month to go for Rio Olympics!

With just a month until the Olympic Games begin, Brazil is unraveling fast -- largely overshadowing the impending arrival of sport's greatest showpiece.

But the world's biggest sporting event is the least of Brazil's worries: against a backdrop of crisis, the Olympics fall somewhere between light relief from political instability, and an unwanted financial burden on a broke Brazilian state.

A crime-plagued Rio de Janeiro, on Monday enters the one-month countdown to becoming the first South American city to host the Olympics. Stadiums are all ready -- barring finishing touches -- and within weeks, Brazil expects to greet at least 5,00,000 tourists for the August 5-21 Games.

The mayor, Eduardo Paes, and Brazil's Olympic committee boss Carlos Nuzman were to give a news conference marking the milestone in Rio's epic effort to transform from a beautiful but crumbling city to glittering stage for the world's most-watched event.

"The city is 100 per cent ready. It's an unbelievable city. I'm very proud of our city," Paes said.

Some 10,000 athletes will compete over 19 days in Rio, ranging from familiar sports icons such as sprinter Usain Bolt and swimmer Michael Phelps to the stars of Olympic newbie sport rugby sevens and golf, which returns after more than a century's absence.

But despite growing excitement in the sporting world and the visible transformation of Olympic sites in Rio, a mounting series of problems are overshadowing the Games.

The authorities will deploy 85,000 police backed by soldiers on Rio's streets.

Terrorism is a serious concern after the Islamic State group demonstrated its geographical reach, with bombings in Istanbul and Baghdad blamed on the group in the last few days.

However, Rio already faces its own serious violent crime that has embarrassed Olympic organizers trying to change the city's image.

Although down from horrific levels a decade ago, the murder rate is on the rise and street crime is also proliferating, symbolized by the hijacking of a truck filled with German television equipment last week.

The mayor and Olympic organizers have been fighting back against a torrent of bad publicity.

Despite the decision of several athletes, notably high profile golfers, to pull out of the Olympics because of fear over the mosquito-transmitted Zika virus, the authorities insist there are no serious health risks.

Zika can trigger birth defects in babies born to infected mothers, but in most cases causes little beyond flu-like symptoms -- and August is a month with few mosquitoes.