Campaigners gathered at the city hall before the march through Cardiff
Campaigners have been voicing their opposition to a new £11bn military training facility in the Vale of Glamorgan.
Building on the centre at St Athan is due to get under way in 2009, under a public-private finance deal.
The academy will provide a central training base for the RAF, Army and Navy bringing thousands of jobs.
However, peace campaigners have called the centre a "school of death", and have been marching through Cardiff.
Plaid Cymru Euro MP, Jill Evans, said she was concerned that the development was being placed in the hands of a private consortium.
""We are opposed to the privatisation of military training and to making Wales the base for that training for Britain and other countries," she said ahead of the demonstration.
"We have the skills and the potential to be a real driving force for peace in the world. Instead the British government is making us the centre of training for war, putting that training in the hands of multinational arms companies."
The Ministry of Defence (MoD) confirmed in January that the training centre would be based at St Athan, led by the Metrix group.
Under the deal, training for 6,500 personnel will be transferred to the Vale of Glamorgan and HMS Sultan, Gosport over five years.
It has been estimated by assembly government ministers that the development will bring 5,000 jobs to the area.
However that figure has been disputed by campaigners, who argue that these will not be new jobs - rather posts transferred to St Athan from other parts of Britain.
But First Minister Rhodri Morgan has strongly defended the project.
"We have worked very hard to win this project for Wales against severe competition from the West Midlands and elsewhere," he insisted.
"The Defence Training Academy will involve activities which are far less military in character, since the academy will involve teaching skills such as engineering, electronics, and IT - all equally as valuable in civilian life after they have left the armed forces as they will be for maintaining military equipment."
Conservative AM Alun Cairns, also backed the centre, saying: "Plaid Cymru have shown their true colours today.
"As part of the ruling administration in the Vale and the Welsh Assembly Government they are supposed to be giving this initiative important strategic support.
"Instead, comments by some of their party's senior figures are undermining that vital work," added Mr Cairns.
Welsh Liberal Democrat leader, Mike German added: "These are highly-skilled jobs being moved into Wales in the face of strong competition from other parts of the UK.
"The real message is that Plaid Cymru want to reduce our armed forces and the valuable role they play in peacekeeping around the world."