Protesters at the previous 'March in May' budget protest in Sydney.

Protesters at the previous 'March in May' budget protest in Sydney. Photo: Louise Kennerley

CBD road closures will be in force early Sunday afternoon as upwards of 5000 people take to the streets to protest the federal government's budget.

Protesters will assemble outside Sydney Town Hall at 1pm before marching along George, Market, Castlereagh and Park streets at 1.30pm.

Leighton Archer from the Transport Management Centre said roads will be closed as participants approach and will re-open as soon as possible, with all roads expected to be clear by 3pm.

Bus services are running to timetable but some will divert from their normal route. Bus passengers can use their tickets on city circle trains between 1.30pm and 3pm.

Mr Archer advised motorists to avoid the CBD during this time and to allow extra travel time.

The national day of action, spearheaded by trade unions, will see simultaneous rallies in Adelaide, Brisbane, Canberra, Melbourne, Darwin and Alice Springs.

Unions NSW Secretary Mark Lennon said the demonstration is timed to target incoming senators ahead of the new Senate's first sitting day tomorrow.

"If new senators allow this budget through in its current form it will fundamentally reorient Australia, making us a less fair society," he said. "They have a critical opportunity to defend mainstream Australian values."

Unions are particularly aggrieved by the budget's cuts to health and education funding, the $7 GP co-payment and deregulation of university fees.

Sydney CBD roads will also be affected by the Reserve Forces Day parade. Macquarie Street is closed until 1pm and there will be rolling road closures on parts of College, Hunter, Phillip and King streets and St James and Prince Albert roads between 10.15am and 1pm.