Hundreds of union members have joined protests in Cardiff and Wrexham against the UK government's cuts to the public sector.
The Treasury has admitted that the savings of more than £80bn could cost nearly half a million jobs.
The Welsh Assembly Government said the Spending Review meant it would lose £1.8bn from its budget over the next four years.
Rallies have also been taking place in other cities across the UK.
Welsh Secretary Cheryl Gillan has said the review was fair, and the assembly government was facing "smaller cuts than most UK government departments".
The public sector employs around a quarter of the workforce in Wales.
The Public and Commercial Services Union (PCS) has about 22,000 members in Wales.
Chair Katrine Williams said: "Our demonstration is intended to show that workers in Wales will not sit idly by and allow our public services to be decimated and the Welsh economy to be wrecked by these vicious cuts.
"Public sector workers face thousands of job losses across Wales, and a huge increase in workloads plus attacks on wages and pensions.
"The jobs of private sector workers will also be hit as the economy shrinks. And we all face the destruction of vital public services.
"Now is the time to stand up and take action against these threats."
The assembly government has said the cuts were at the lower end of predictions, but the budget was the "worst since devolution".
Deputy First Minister Ieuan Wyn Jones has said Wales could see up to 25,000 public sector job losses over four years with the same number in the private sector.
Welsh Secretary Cheryl Gillan has called the review a "fair funding settlement for Wales".
She has said: "The Welsh Assembly Government is facing smaller cuts than most UK government departments but, like everywhere else, tough decisions will have to be faced in Cardiff Bay."