Police have arrested 13 people after a group of environmental activists abseiled off the Sydney Harbour Bridge to demand the major federal political parties declare a "climate emergency".

About 5.15am on Tuesday, Harbour Bridge security staff spotted several people on the bridge.

They detained some while waiting for police to arrive, but part of the group climbed under the bridge and begun abseiling, a NSW Police Force spokesperson said.

Channel 7

SYDNEY HARBOUR BRIDGE: Lane 8 southbound (Cahill Exp) is closed on the bridge due to a police operation. Exercise caution. — Live Traffic Sydney (@LiveTrafficSyd) May 13, 2019

Police arrested 10 people earlier this morning before arresting the final three, who were attached to ropes beneath the bridge, about 9.30am, bringing the operation to a close.

"Australia is facing a climate emergency right now," Greenpeace Australia Pacific CEO David Ritter said in a statement on Tuesday.

Greenpeace

"We're here today to say 'no more'."

#BREAKING: @Greenpeace activists have scaled the Sydney Harbour Bridge. They’re calling for @ScottMorrisonMP to declare a climate emergency. Some protestors have been arrested. @7NewsSydney pic.twitter.com/2at1QIpk5u — Natasha Squarey (@NatashaSquarey) May 13, 2019

The Greenpeace supporters unfurled banners calling for action as they abseiled from positions underneath a public walkway on the western side of the bridge.

Roads and Maritime said one lane of the bridge was closed earlier, during the police operation, but motorists are no longer expected to face delays.

Police said all 13 protesters are expected to face charges.