Unison is surveying members in Scotland about staff abuse after Paul Kelly, its convenor for the community and voluntary sector in Scotland, said he believed the problem had worsened.
"Staff are under continuous threat of violence, intimidation and verbal abuse, and management's answer to the problem is often to fill in incident forms," he said.
He added that managers were reluctant to tackle the issue because of the financial implications of dismissing staff.
"If there is a zero-tolerance policy in local government, why shouldn't the same apply in voluntary sector projects funded by local authorities?" he asked.
Mike Short, national officer for the community and voluntary sector at Unison, said bullying was "a huge issue" across the UK charity sector.
But Calum MacAulay, convenor for the Association of Chief Officers of Scottish Voluntary Organisations, said working with vulnerable clients put strain on staff but charities did take bullying seriously.
"Some voluntary sector clients have behavioural difficulties that may cause them to be challenging to those who work with them," he said.
"Any zero-tolerance style of approach must be sophisticated enough not to punish people inappropriately."
Unite, formerly Amicus, is running a Dignity at Work campaign to combat workplace bullying. Rachael Maskell, national officer for the union, said bullying was "a big issue" for charities and was partly due to financial stresses within the sector. "Operating on thin margins creates a lot of pressure," she said.
Management failure is more likely to cause bullying in the voluntary sector than it is in the rest of the economy, a survey has shown.
A poll of 500 workers by the Chartered Management Institute and anti-bullying charity the Andrea Adams Trust revealed that 72 per cent of people in the charity sector thought that a lack of management skills contributed to bullying in the workplace. The comparable figure for the economy as a whole was 66 per cent.
Jo Causon, director of marketing and corporate affairs at the institute, said: "Poor management is often at the root of the problem, because staff at all levels lack the skills to tackle the issue."
Abby Wright, director of membership at chief executives body Acevo, said charity managers were striving to improve the quality of leadership in the sector.