Spain shrugged off a security scare that meant they had to start their game in an empty arena as they registered their first basketball win of the Rio Olympics on Thursday.

Spain, which won silver in 2008 and 2012, trailed underdog Nigeria 66-65 to start the fourth period, but balanced scoring down the stretch from veteran Olympians including Pau Gasol and Rudy Fernandez helped them avoid a third straight loss.

Gasol finished with 16 points and Fernandez 13.

The game started with no fans present after Brazilian security forces prevented spectators entering the arena and then staged what appeared to be a controlled explosion just outside.

A loud bang was heard just before tip-off of the game in Rio's main Olympic park.

Security officials at the Olympic park had told AFP just before the explosion that there was a "special situation" preventing them from allowing fans inside.

However, fans were allowed in soon after as security gave the all-clear. The game was unaffected.

Spain, which is ranked number two in the world and was bridesmaid in the past two Games to the powerful USA squad of NBA stars, built an early lead over the lightly regarded Nigerians.

But Nigeria closed the gap on the strength of 13 three-pointers including seven by shooting guard Chamberlain Oguchi.

The win helps to keep alive Spain's hopes of advancing to the medal rounds despite last-second losses to Croatia and Brazil to open the competition.

They are now 1-2, while Nigeria drops to 0-3.

A Rio 2016 spokeswoman at the arena declined comment on the security scare, saying Games organisers would issue a statement.

There have been at least three controlled explosions of suspect items in Rio since the Olympics started. One was near the finish line of the cycling road race on Saturday.