A police officer has been filmed allegedly punching a teenage girl in the head during an arrest at a pro-marijuana rally in central Melbourne.

Footage of the incident emerged on Saturday after the 15-year-old was arrested at the rally in Flagstaff Gardens about 2.30pm.

Footage has emerged of a police officer appearing to strike a 15-year-old girl after allegedly being spat on at a Melbourne rally.

Witnesses said the teen was pulled to the ground by officers during her arrest. When she was eventually led away, she allegedly spat in the face of a female officer, who appears to retaliate with a right hook.

The girl was charged with trafficking drugs, possessing drugs and assaulting an officer. She is due to face the Children's Court at a later date.

Footage has emerged of a police officer appearing to strike a 15-year-old girl in the head after being spat on. Supplied

A spokeswoman for Victoria Police said on Sunday police had not received a complaint about the officer and would only investigate if one was received.

Victorian MP Fiona Patten, who spoke at the rally, has called for an urgent investigation into the incident.

"For police to get violent in this manner and use that type of heavy, over-handed tactics, we really need to question that and question the priorities of police", she said.

"I would hope that sort of violent behaviour is not sanctioned by police and is fully investigated."

Ms Patten said the police presence at the rally seemed "overwhelming and unnecessary".

"It's a peaceful demonstration – there's no need for police action we're seeing today," she said on Saturday.

Marijuana enthusiasts at the event. Paul Sakkal

Witness Greg Reiner said he saw about 10 police surrounding the girl soon after she was arrested.

"She was struggling to walk because the officers were manhandling her quite violently," he said. "Then about halfway through the walk out we saw she got punched in the face."

Cries of "Leave her alone, she's a kid" can be heard in the footage as the officers hold the teen on the ground.

As the officers walked her from the park, they were followed by what bystanders have described as an "angry mob" of onlookers, chanting and booing at them.

"A whole mob then followed the police and the girl as they walked her out, probably 300 people," Mr Reiner said.

"A few of us had to convince the mob to turn around and stop following police because the mob were really angry at how this girl was treated."

The annual 420 rally is held worldwide on April 20 by marijuana enthusiasts to campaign for the liberalisation of drug laws. The rally takes its name from the time 4.20pm, when all attendees smoke marijuana in unison.

Greens MPs Adam Bandt and Tim Read, and Liberal Democratic Party MP David Limbrick spoke at the rally along with Ms Patten.

Thirty people were issued a caution for cannabis possession at the rally. One person, who was arrested in relation to possessing cannabis and trafficking a drug of dependence, will be charged on summons.

Acting Inspector Lisa Prentice-Evans said "police will facilitate peaceful demonstrations but if there's breaches of legislation we'll take proactive enforcement either by way of diversions, cautions, or through the courts".