The body set up to monitor care homes and private clinics received more than 12,000 complaints in its first year.
One in 10 of these were allegations of abuse by staff.
The National Care Standards Commission, which was established in April 2002, also investigated 3,583 complaints about poor care.
In its annual report, it says one in four complaints was upheld. It ordered urgent improvements to be taken in 2,600 cases.
The commission is responsible for inspecting social care and private and voluntary health care services throughout England.
These include children's homes, care homes, private hospitals, adoption and fostering agencies and services.
1. Poor care - 3,583 complaints
2. Staffing - 2,896 complaints
3. Abuse - 1,278 complaints
4. Premises - 991 complaints
5. Food - 880 complaints
Source: National Care Standards Commission
The vast majority of complaints concerned adult services.
While many of these complaints related to poor care, the commission also investigated allegations that centres were not staffed or managed properly.
It also examined claims that some premises were unsatisfactory and that some were serving poor quality food.
The annual report highlights a number of cases investigated by the commission in its first year.
This included an investigation into a children's care home in Manchester. The home accommodated seven young people with behavioural problems between the ages of 12 and 17.
Inspectors found that the home was in a very poor state of repair and that staff could not manage the children.
Some bedrooms had holes in the walls, toilet facilities were unsafe and the home was not meeting fire safety standards.
The home failed to comply with an action plan and was closed down.
Inspectors also investigated the case of an 80-year-old woman who had been strapped to a chair with a dog leash in a care home. Social services were called in and the woman was transferred to another home.
The commission has the legal power to order care homes and other centres under its remit to make urgent improvements if they are found to be in breach of standards.
It can prosecute owners if they fail to comply and can also order centres to be shut down. In its first year, the commission shut four centres.
In its report, the commission says many people are afraid to complain about poor standards of care.
"We know that people using private health and social care services are often not aware of their rights and are too afraid to complain if the quality is below standard.
"We will campaign to raise awareness of people's rights during our second year."
The National Care Homes Association said it was disappointed with the performance of the commission during its first year.
A spokeswoman said inspectors were inconsistent and some were heavy handed in their approach.
"Too much attention is sometimes paid to paperwork and not enough is being paid to care," a spokeswoman said.
"We had hoped for a much greater drive towards consistency."
Meanwhile, the Department of Health has announced that care homes must carry out stringent background checks on their existing staff.
New care workers already have to be checked by the Criminal Records Bureau. This is now being extended to existing agency or residential and nursing home staff.