The four cities vying to host the 2016 Summer Olympics are braced for a close call when the IOC chooses the winner in Copenhagen next week.

Tokyo, Madrid, Chicago and Rio de Janeiro have been engaged in a tight battle, with no clear front-runner emerging after two years of frantic campaigning.

"We are heading for what I believe will be a fascinating 10 days in Copenhagen," said International Olympic Committee chief Jacques Rogge. "We are very thrilled that the four candidates are first class. Each of them could organise a very good Games."

The IOC will decide the host of the world's biggest multi-sports event during its session on October 2, with Rogge predicting a narrow win when the 115 members cast their votes.

"It is probably going to be a couple of votes," Rogge said earlier this month. "Two, three, four votes... it is going to be very close. Don't forget that something like four, five votes is a change of mind of just two or three people."

While past bid races going into the final vote have had a favourite, who may or may not have won in the end, this time the cities are locked in a neck-and-neck race.

Chicago was seen as being ahead for much of the campaign but rows between the IOC and US Olympic Committee over revenue sharing and now-postponed plans for an independent Olympic television station in America has not done the bid any favours.

Its budget, largely dependent on private funds during the economic crisis, is also looked upon with some scepticism.

Chicago's initial hesitation in signing a host city contract that would legally bind it to cover any deficit arising from the Games was noted in an IOC evaluation report this month.

However, the presence of US President Barack Obama, who has spent much of his political life in the city, and his wife Michelle in Copenhagen to back the bid, will help.

A US broadcasting rights deal for the 2014 and 2016 Olympics, the IOC's biggest single source of revenue, has yet to be signed and choosing Chicago would boost the final figure as American TV would expect bigger ad revenues from a "home Games".

Rio has taken considerable strides in its efforts to land the first Games for a South American city.

With football's world governing body FIFA giving the World Cup finals to South Africa in 2010 and Brazil in 2014, the IOC is quickly running out of excuses for not handing the Games to a city south of the equator that is not in Australia.

Nevertheless, the IOC has expressed concerns that the World Cup could cut into the 2016 Olympics' marketing and promotion success if the Games went to Rio and the city's notoriously high crime rate would also need to be tackled.

A similar time zone to North America is an advantage for Rio, though, because US broadcasters dislike the big time differences in Europe and Asia that dilute the impact and financial punch of staging the Games at home.

Tokyo, host of the 1964 Olympics, has already won praise from Rogge for its solid financial plan and the high technical standard of the bid.

Sending the Games to Asia only eight years after the 2008 Beijing Olympics remains one of the bid's drawbacks, though.

IOC opinion polls conducted this year also showed relatively low public support, although Tokyo has said fresh results indicate much stronger enthusiasm for the Games.

Madrid is pinning its hopes on an improved bid, after failing to win four years ago, but a second European Games after London 2012 has not worked in its favour.

The city can, however, take heart from the on-site support of Prime Minister Jose Luis Rodriguez Zapatero and King Juan Carlos which will be an important diplomatic lever.