The government was accused of basing its draft Mental Health Bill on a "tabloid agenda".
Campaigners at a seminar said the bill had been influenced by stories about mentally ill people who had committed murder and this had "swung the argument" away from the rights of mentally ill people.
Paul Farmer, chair of the Mental Health Alliance, called for a tightening of the proposals that extend powers of compulsory detention.
Lord Denis Carter, a member of the joint committee on the draft Mental Health Bill, said the government had "no idea" of how many more people would be compulsorily detained.
But health minister Rosie Winterton said the government had made it clear that compulsory orders would be aimed at people who were in and out of treatment services. Its estimate of numbers to be treated under the draft bill's proposals was based on people in this category, she added.
