Bernard Higgins said the 12-day COP26 event later this year, and expected environmental mass protests alongside, made it a "very complex and challenging operation".

Up to 120 world leaders, including US President Joe Biden and Pope Francis, are due to attend, as well as senior British royals, and thousands of delegates.

Police Scotland has said some 10,000 officers drafted in from across the country will be deployed each day during the conference, which runs from October 31 to November 12.

"This is certainly the biggest police operation that has ever been in Scotland and potentially the UK," Higgins told an online briefing of the Scottish Police Authority.

"It will attract the greatest number of world leaders ever in one place in the UK. So it is a very complex and challenging operation."

Higgins, an assistant chief constable at Police Scotland, said a planned climate rally with 50,000 to 100,000 people in Glasgow had added to the logistical difficulties.

"You lump them all together and that tells you what sort of fortnight we are going to have," he added.

The assistant chief constable has met leaders of the Extinction Rebellion protest group, whose direct action has brought parts of London to a standstill in recent weeks.

He said he explained police would facilitate peaceful protest and "unlawful protest to a point".

"Our fundamental approach here is facilitation, engagement, it's fair and friendly," he said.

"But when the protest starts to endanger others, or when the protest starts to impinge on the ability of conference to operate then will might have to increase our reaction to it," he said.

Hundreds of officers have been given specialist training in public order tactics in preparation.

A range of units will be in place, including armed officers, mounted police and dog units, marine police and air support, with support from police forces around Britain.

On Tuesday, the Climate Action Network, a global collection of more than 1,500 environmental groups, called for COP26 to be postponed.

They cited inequalities in the global Covid vaccine roll-out and stringent quarantine requirements for dozens of countries seeking to attend the talks.

But the United Nations and hosts Britain said the conference would go ahead, as urgent action was required to set emissions targets to prevent runaway climate change.