Alan Lau Yip-shing, former deputy commissioner of Hong Kong police recalled from retirement to help deal with anti-government protests, lambasted the relentless smear campaign that has partly turned public perception of the police force from one of the finest in the world to a brutal force of oppression in just a few months.

In an internal letter to the force, Lau lauded the discipline and dedication of his fellow officers in these trying times.

Calling it an honor to serve alongside his fellow officers, Lau wrote that the past three months of civil unrest posed a challenge that was the most "difficult and worrisome".

He added that it was something that tested the resolve of Hong Kong's 30,000 police officers.

The letter from the former deputy police commissioner, who oversaw operations during the illegal "Occupy Central" protests in 2014 and the Mong Kok riot in 2016, has been hailed as a timely attempt to boost the morale of the police force maligned by outrageous lies and rumors circulating on social media and fanned by anti-government politicians and activists.

Lau praised the police as being the bulwark against the savage onslaught on the city's law and order.

In the past three months, protesters have sabotaged the city's mass transit system, undermined the special administrative region government's authority tried to influence impressionable young students.

"It would be hard to fathom what would happen to the city if it weren't for your perseverance and valiance." Lau told his fellow officers.

He seemed particularly incensed by the false accusations leveled at the police.

"We are framed," said Lau, adding that the lies and rumors against the police were mostly fabricated by those with axe to grind.

They besmirched the police force in an attempt to tear the society apart by sowing suspicion and distrust in families, the workplaces and among friends, said Lau.

It is an urgent task for Hong Kong to uproot the evil of hatred and intolerance, Lau said, adding that it could only be done with the support and cooperation of people from all walks of life.

Lau has been appointed to oversee police operations at large-scale public events, including the activities to mark the 70th anniversary of the People's Republic of China on Oct 1.