More than 100 people met at the corner of Collins and Swanston streets to rally against Mantle Mining's licence to drill for coal and its long-term plans to start an open-cut mine in Moorabool.

The federal Greens, Environment Victoria and Friends of the Earth joined Moorabool Environment Group to condemn Mantle's exploration licence, which covers a 38,000-hectare area from Bacchus Marsh to the outskirts of Ballan.

"Back off from Bacchus ... farms not fossil fuel" was chanted across the busy CBD intersection before the rally marched up Collins Street to the offices of Mantle's new financial backer, Cygnet Capital.

In a statement to the stock exchange last week, Mantle said Cygnet was now raising equity for initial drilling programs at 15 sites around Bacchus Marsh. Last week, the former underwriter, Intersuisse, withdrew its backing from Mantle Mining for undisclosed reasons.

With a strong police contingent close behind, activists and angry Moorabool residents crammed into a small lobby, some chaining themselves together, outside Cygnet's 10th-floor offices.

But Cygnet general manager Mark Davis said the protest had not affected his decision to underwrite Mantle Mining.

The rally disbanded after an extra 15 police arrived and told protesters to leave. Four refused and were arrested.

A Victoria Police spokeswoman said they had since been released and will later be charged with trespassing.

Greens Senator Richard Di Natale told the Weekly he hoped the protest would make Mantle change its mind. He said objectors could be optimistic after Mantle scuttled similar exploratory plans in the Otways when faced with a public backlash.

"Mantle has already shown it's vulnerable to community protest and there's no question that Bacchus Marsh doesn't want this," he said.

Senator Di Natale said Mantle's plan to eventually establish an open-cut mine "goes against the grain" of the interests of farmers, residents and environmentalists.

"We want prime agricultural land protected and our broader environment protected by making sure we won't increase our emissions."

But Energy Minister Michael O'Brien said he supported Mantle's exploration licence.

"I am working to ensure a sustainable mining industry in Victoria and balancing the interests of all communities, particularly farmers," he said.