Portland police have said one of their lieutenants 'firmly believed' that some milkshakes being thrown at protesters during a rally were filled with concrete.

Lt. Richard A. Stainbrook wrote a report containing the claims about the June 29 protests in Portland, Oregon, two days after they occurred, reports the Oregonian.

During the weekend rally, group Popular Mobilization are said to have organized a milkshake party aiming to be a counterprotest against the right-wing Proud Boys.

The police force tweeted there had been 'reports of individuals throwing milkshakes' with a substance mixed in that was similar to a quick drying cement' on the day.

Portland police have said they believed some milkshakes being thrown at protesters during a rally were filled with concrete. Pictured is conservative writer Andy Ngo who was 'attacked by Antifa' and had a drink thrown at him during the rally on June 29

Mr Ngo also shared a picture of his bag covered in what looked like a milkshake at the rally

Portland police did not specify in their original tweet or in the report later released which group at the demonstrations allegedly threw the milkshakes.

Death threats were made against Popular Mobilization, who are also known as PopMob, after the tweet was posted.

They claimed that the only ingredients in the drinks were coconut ice cream, cashew milk and rainbow sprinkles.

In his report Lt. Stainbrook explained how he had been approached by a woman whose head and shoulders were covered with a gray-colored substance.

He said that he had previously worked with concrete and recognized the substance as Quikrete.

Lt Stainbrook wrote: 'I have worked with concrete periodically and specifically QUIKRETE, which is pre-mixed concrete, specifically many times.

'The substance on the female smelled like QUIKRETE. I also noticed as the substance was drying it was turning into a chalky consistency which from my experience is consistent with drying concrete.

Portland police have released Lt. Stainbrook's report of the rally in which he says he thought the substance on a woman was Quikrete and broadcast this on his radio

'I firmly believe these 'Milk Shakes' that were being thrown around and on people contained some form of concrete.

'For situational awareness purposes, I broadcast over my radio that the 'Milk Shakes' being thrown around contained Quikrete.'

Quikrete is a mix of gravel, sand and cement and can be used for various things including building or repairing foundation walls, sidewalks, curbs and ramps.

Police Chief Danielle Outlaw said the bureau shared the information 'in good faith' and that there wasn't any 'ill intentions' by sharing the tweets.

Police quickly shared the information that milkshakes could contain quick-drying cement on Twitter but faced criticism after the event

Officers also advised people to get out of the street and said police had been hit by eggs

Conservative writer Andy Ngo, an editor at the online publication Quillette, was also attacked at the June rally.

Video from the day shoes him being punched, kicked and covered in milkshake by members of the 'Antifa' movement.

The writer tweeted afterwards that he was bleeding and had been robbed of his camera equipment and was heading to the hospital for treatment.

He said: 'Attacked by antifa. Bleeding. They stole my camera equipment. No police until after. waiting for ambulance . If you have evidence Of attack please help.'

Adding: 'On way to hospital. Was beat on face and head multiple times in downtown in middle of street with fists and weapons. Suspects at large.'

Multiple demonstrations were planned for June 29, with one involving the right-wing group the Proud Boys and planned counter-protests involving a local Antifa group

It is thought that the clashes occurred when opposing groups met in the streets. Pictured is one group protesting through Portland

At least three demonstraters from Antifa, the term for a militant progressive protest movement, who oppose the far-right, were arrested during the event.

It is thought at least three groups had planned rallies or demonstrations at different sites in the city.

The fights occurred when participants of the opposing groups met, according to The Oregonian/OregonLive.

The protests came a day after white supremacist James Fields Jr. was sentenced to life in prison in Virginia.

He had driven his car into counter-protesters of a 'Unite the Right' rally in 2017, killing one and injuring dozens more, reports USAtoday.