Police have provided no evidence that antifascist protesters threw milkshakes mixed with quick-drying cement during violent clashes in Portland over the weekend.

Conservative writer Andy Ngo was one of a number of people injured when he was attacked by masked antifascists during demonstrations in the Oregon capital on Saturday.

Members of Antifa were counter-protesting a march in the city by the Proud Boys, a male chauvinist group known for street fighting.

The Portland Police Bureau (PPB) said on Saturday it had received reports of individuals throwing “milkshakes” with “a substance mixed in that was similar to a quick drying cement”.

The claim quickly generated headlines in the US, but the bureau failed to provide any evidence and it emerged the claim was based on one officer’s assumption, according to the Portland Mercury.

The vegan milkshakes were reportedly provided by a left-leaning group called Popular Mobilization, which handed out hundreds of the drinks to demonstrators. A number were thrown at officers and protesters associated with the far-right.

"A Lieutenant in the field broadcast information of his observation of a cup which appeared to have material on it consistent with quick drying cement," PPB spokesperson Tina Jones told the local newspaper.

"We put out the tweet to bring attention to this potential hazard and to encourage people to contact us if they were the victim of a crime."

A number of people who said they drank the milkshakes posted on Twitter insisting they were not mixed with anything else.

“Milkshaking” right-wing figures started in the UK last month, when the British anti-Islam activist Tommy Robinson was targeted, and has since spread to the US.

The trend has led to an increasingly polarised debate as to the limits of legitimate protest, and whether throwing drinks at divisive political figures could lead to more dangerous tactics in future.

Local media reported that only 30 people turned up for the original protest – a “patriot prayer” rally – in Portland’s Pioneer Courthouse Square on Saturday afternoon.

Police attempted to keep them apart from a larger counter-demonstration in Lownsdale Square, but disorder broke out when a group of antifascist protesters left their designated area and marched towards the Proud Boys rally, repeatedly trying to get around police blockades.

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One banner read “f*** Nazis and fascists”, while some protesters waved a banner for the Satanic Portland Antifascists and chanted: “Not hate, no fear, Proud Boys are not welcome here.”

Footage showed scuffles between opposing protesters, while several antifascists were wearing face coverings or helmets, and carrying homemade shields, bats and weapons.

PBB said two protests from Chapman Square and Lownsdale Park Square merged and started an unpermitted march towards Pioneer Square that stopped traffic.

A right-wing protester after being attacked in Portland's Pioneer Courthouse Square on 29 June (Moriah Ratner/Getty Images)

“There were multiple assaults reported, as well as projectiles thrown at demonstrators and officers,” a statement added. “There were also reports of pepper spray and bear spray being used by people in the crowd. Officers deployed pepper spray during the incident.”

Two officers were pepper sprayed, another was punched and a colleague was hit in the head with a projectile during what police labelled a “civil disturbance”.

After issuing a dispersal order, three people were arrested. A 23-year-old man was charged with second-degree assault and assault of a police officer, a 21-year-old man was charged with harassment and a 23-year-old woman with disorderly conduct and harassment.

Assistant police chief Chris Davis said: “There are hundreds of peaceful free speech events in the city in a given year that do not result in violence. Unfortunately, today some community members and officers were injured.

“We are actively investigating these incidents to hold those responsible accountable.”

Police are appealing anyone who witnessed or filmed violence to contact crimetips@portlandoregon.gov.