A killer whale stranded in a river in the Japanese city of Nagoya has returned to the ocean, cheered on by thousands of spectators.

The young male orca whale swam back to open sea on Wednesday after local authorities and aquarium officials tapped metal rods in the water in the hope that it would swim away from the noise.

Progress stalled at one point when the whale had trouble negotiating around a bridge support, and rescuers worried that it might become exhausted.

But the whale appeared to be in good condition after the ordeal, port spokesman Kameaki Mizuno said.

"It's swimming around and seems to be okay," Mr Mizuno said.

The 5m-long (14ft) whale swam into the Horikawa River in Nagoya, 269km (168 miles) west of Tokyo, on Tuesday.

Local aquarium officials said it may have been attracted by the city lights after losing touch with its pod, or family, while chasing a school of fish near the river.

City celebrity

The young whale brought the city to a standstill - traffic gridlocked as residents flocked to the riverbank to catch a glimpse of the aquatic visitor.

The whale made a splash with the Japanese while stuck 4km (2.4 miles) upriver, performing a few jumps for the excited crowds.

Smaller animals such as porpoises are sometimes found in the river, but whales rarely make their way upstream.

"We've never had anything as big as a killer whale in the river," said Aichi government spokesman Kenji Tamura.

Killer whales, which have a black back and white underbelly, have a reputation as ferocious hunters. They grow as large as 9.5 metres and live in pods.