Union leaders representing thousands of staff in Southern Cross care homes staged a demonstration today outside a meeting attended by private equity companies.
The GMB said the target of its protest in central London was Blackstone, which bought Southern Cross in 2004 before selling the firm later.
Southern Cross, responsible for looking after 31,000 elderly residents, has announced that it will underpay its rent from this week as it struggles with a £230 million annual rental bill.
The firm, which owns almost 750 care homes and employs thousands of staff, will pay nearly a third less rent than it is obliged to for the next four months in what is effectively a loan from its landlords.
Officials said there was no threat of imminent closure of any of the homes but union leaders, and families of residents, called for urgent action to help ease mounting concern.
The GMB protesters carried flags and banners with slogans such as "Private Equity More Secret Than The Mafia."
General secretary Paul Kenny said: "These kings of private equity meet in secret, excluding the press and the public, what further ravages are they planning for the British economy and British jobs?
"More is known about the Mafia than about the antics of private equity. It is time to shine a very bright light on their activities.
"This is a financial exploitation process that does not care if it exploits elderly people in care, motorists by the side of the road or customers and tied tenants in pubs.
"GMB is calling on the politicians to put a stop to this. The tax breaks on the debts private equity use have to be removed. The self regulatory code is a complete fiction and is useless."