A spokeswoman for Leonard Cheshire said it served 15,000 people with disabilities across the country, providing personal as well as residential care.
About 20 people, some in wheelchairs, noisily blockaded the entrance of an office block and emptied bags of red and white feathers over the floor.
The target was voluntary sector charity Leonard Cheshire, which has reacted with surprise to the attack and strongly denies accusations of mistreatment.
The same demonstrators had earlier hit the Department of Health in Whitehall, where they reportedly daubed brown paint on the front of the building.
The group behind both protests is the radical Disabled People's Direct Action Network, or DAN.
It is seeking "equal treatment" for disabled people and an end to local authority charges for personal care.
A spokeswoman for Leonard Cheshire said it served 15,000 people with disabilities across the country, providing personal as well as residential care.
She said it had received "no petition or any communication at all" from the protesters.
Call for end to charges
Members of DAN had earlier blockaded the Department of Health in protest at charges for personal care.
Simone, one of the demonstrators, said they were demanding an end to charges for "essential services", like assistance getting out of bed.
She said: "Why should disabled people be charged for services?
"Disabled people pay taxes like anyone else."
Another protester, Claire, suggested disabled people were restricted by rules preventing them taking their package of care services with them if they moved to a different area.
She said: "We are not asking for something special.
"We want to be treated equally."
It is currently up to the discretion of local authorities whether to levy means-tested charges for personal care.
Charging policies consequently vary from one part of the country to another, prompting campaigners to describe the system as a "postcode lottery".
Liberal Democrat spokesman on social services Paul Burstow MP said personal care should be free based on an assessment of people's needs, and not ability to pay.
Mr Burstow said there needed to be "far greater consistency and clarity in what local authorities can charge for".
Consultation
The government is attempting to regulate home care charging policies on a national basis and has almost finished a consultation on new draft guidance.
A Department of Health spokesperson said existing charging policies could act as a "disincentive" to work.
She said: "Some severely disabled people who work can find that most of their earnings are taken up by paying for intensive care packages."