Almost 200 people were on board the Flemington-bound train when 20 protesters flooded the tracks, letting down the tyres of the vehicle near a level crossing in Ascot Vale about 1.15pm.

Metro Trains said 55 people disembarked and walked along rail line in response to the delay, creating a "significant" safety risk.

Police and firefighters were forced to extract the 27-year-old woman from the locked car before she was arrested.

She had been tethered to the steering wheel with a bike lock around her neck.

The woman, who wanted to be known only as Justine, told The Age she didn't feel unsafe because protesters had warned a train driver prior to their action. Metro denies it was informed.

"I don't really think it was a scary position in comparison to the position people are in on Manus," she said.

The woman, from Moonee Ponds, is expected to be charged with reckless conduct endangering life, careless driving and unlicensed driving.

But she said the action had been worthwhile: "Responding to torturous treatment of innocent people ... to whatever small degree is not unreasonable to me."

Victoria Police Superintendent Tim Hansen said it was a "really dangerous situation".

The protest was one of two Manus-related actions planned to interfere with Melbourne Cup festivities.

In the other, two women scaled a crane near the main grandstand, unfurling a banner that read: "SOS: Evacuate Manus Now!"

The action comes as 600 asylum seekers and refugees remain stranded on Manus Island.

The men have barricaded themselves in the now-closed detention facility and are refusing to move into the community because they fear they will be attacked by locals.

Food, water and electricity have been cut off.

Refugee rights advocates released a statement soon after the train track protest, condemning the Melbourne Cup as animal cruelty and criticising the Australian government for cutting off supplies to the Manus Island detention centre.

"Melbourne celebrates animal cruelty while refugees suffer," spokeswoman Sonia Arquette said.

"We are calling on the Australian government to bring the refugees to safety. We want to see an end to the torture of people and animals."

Inside Flemington Racecourse, Hannah Patchett, 24, and Katherine Woskett, 27, spent about four hours hanging from a crane to denounce the government's treatment of asylum seekers and refugees.

Charlotte Lynch from co-ordinating group Whistle-blowers, Activists and Citizens Alliance said the women descended about 3.30pm - just after Rekindling claimed the Melbourne Cup - when police agreed there would be no arrests.

The group has now warned of "ongoing and escalating" action.

"There was no objective in staying longer," Ms Lynch said. "The sooner we come down, the sooner we can climb the next tall crane. We will be continuing to escalate our actions until Manus is evacuated."

The women have prior form disrupting public events.

Last year, they scaled the 162-metre Arts Centre spire during White Night to protest indefinite detention on Manus Island and Nauru.

The duo carried out a similar stunt, hanging from an overpass on the Eastern Freeway during morning peak-hour, in the same month.

Ms Lynch said the women were never in danger, and held qualifications as a rope access technician and an arborist.

"Those women are absolutely fine. We are genuinely concerned with the safety of the men on Manus," she said.

A Victoria Racing Club spokeswoman said the VRC condemned "in the strongest terms" the "reckless protest action".

"It is now a police matter," she said.

"We thank the police, emergency services, transport authorities and the VRC staff for their thoughtful handling of this situation, on this day of national unity and celebration."

Melbourne Cup itself was relatively trouble-free.

Victoria Police said at 9.30pm there had been 33 evictions from security, six people arrested for being drunk, one for assault, four issued with penalty notices for offences under the Major Events Act, one arrest for offensive behaviour and five for other offences.

One woman was arrested outside the racecourse on Epsom Road about 9.30am, after assaulting a man protesting animal cruelty.

Superintendent Hansen said three women damaged a banner and assaulted the man. The woman was interviewed by police and released pending summons.

Police are still looking for the other two offenders.

With Mel Cunningham, Joe Hinchliffe