MPs and union leaders were meeting at the Cadbury Club in Bournville
Cadbury staff from across the UK and from Dublin are meeting in Birmingham to show their opposition to the hostile takeover bid by US firm Kraft.
Members of the union Unite and MPs are unveiling a petition against the move, at the Cadbury Club in Bournville.
Cadbury, which owns brands such as Dairy Milk and Wispa, has rejected Kraft's bid, calling it "derisory".
Shareholders now have until 5 January to respond to the offer, which Kraft said was in the interest of both firms.
Unite has said it will contact all shareholders to urge them to reject Kraft's bid.
On Wednesday, the petition will be taken to the House of Commons.
Unite's assistant general secretary Len McCluskey said there was worry among staff as to what the Kraft offer could mean for Cadbury.
"Cadbury is a great UK success story - and it was and is not for sale," he said.
"But suddenly, a hostile bid and swarming speculators has thrown its future, its investments plans and the jobs of thousands of workers here and in Ireland up in the air."
Unite met with Kraft in November and said the firm did not give any guarantees over the workforce.
Cadbury has plants in Birmingham, Dublin, Herefordshire and Somerset.