The intervention comes as more than 150 organisations across the UK, including Oxfam, Christian Aid, Shelter and the Salvation Army, rally against the initiative under the banner Keep Volunteering Voluntary.
Community work placements are key to the Government's £300 million Help to Work programme announced by the Chancellor George Osborne last autumn and which was introduced at the end of April.
Under the scheme, long-term jobseekers are asked to sign in daily at a job centre, undergo intensive coaching to find a job or undertake unpaid work in their community, working 30-hour weeks for six months.
Claimants who do not comply face having their benefits cut.
Jobs would include working in charity shops or community cafes, cooking meals for the elderly or picking up litter.
But Voluntary Action Scotland, which represents 32 local area support groups for charities, said the community work placement scheme would coerce people into "mandatory" volunteering because of the financial sanction if they don't take part.
In a statement the group said: "We believe firmly that volunteering must be a free will activity. It is done without coercion and not for financial gain. It is a public and civil good which is highly valued by the volunteer and those benefitting from the volunteer's efforts. It is for these reasons that we are growing increasingly concerned about the introduction of community work placements."
Calum Irving, chief executive, of Voluntary Action Scotland, added the scheme would create different levels of volunteers and that charities would receive no funding despite the extra work involved in administering it.
"The scheme has the potential to create conflict within an organisation as some volunteers will be there through their own choice, believing whole heartedly in the role they are undertaking, while others may be there under a 'community placement' and therefore not by choice," he said.
"It could lead to resources being diverted from those currently volunteering to supporting individuals on community work placements within volunteering organisations. In this instance both the volunteers and those benefitting from volunteer led activities will suffer."
He added that he believed while volunteering in general could help people find paid work, the scheme being proposed was not the right way to proceed.
Mr Irving has written to Iain Duncan Smith, the Work and Pensions Secretary, outlining his concerns and urging him to rethink the plans.
Voluntary Action Scotland's arguments have also been supported at the Scottish Parliament, where a motion put down by Kevin Stewart, SNP MSP for Aberdeen Central, has been signed by 27 MSPs.
Mr Stewart said: "Forcing people into volunteering undermines the ethos of volunteering being a free-will activity and may cause charities and third sector real problems in having to accommodate people who may not want to take part in the activities that they are tasked with.
"This is yet another example of the Westminster Government enacting policies that are ill-thought out and which undermines the entire ethos of volunteering."
John Downie, director of public affairs at the Scottish Council for Voluntary Organisations, said the scheme was badly thought out.
He said: "Community work placements completely miss the point that volunteering should be something people choose to do. Instead, what we have here is yet another expensive programme which punishes people for not having a job and which will do little to help them or the charities and communities in which they are forced to give their time for free.
"There are many better alternatives which have been proven to help unemployed people."
A DWP spokesman said community work placements were designed to help the long term unemployed back to work and give jobseekers employment experience and opportunities.
The placements will be backed up by supported job searching and that the scheme had received positive feedback when piloted, he said.
He said: "Charities and community based organisations are under no obligation to take part in the scheme, but many are because they recognise the positive benefits of this type of provision both for their own aims and objectives within the community and for claimants who can benefit from a period of work experience and supported job search. Evaluation of our Trailblazer showed CWP had a significant positive impact on jobseekers' confidence, motivation, and work-related ambition."