Thousands of workers have converged on central Melbourne as unions mobilise nationally against the Abbott government's workplace relations agenda.

A massive crowd marched from Carlton to Federation Square on Wednesday afternoon, blocking traffic at several major intersections and causing parts of the CBD to grind to a halt.

"They say cut back we say fight back," organisers chanted through the sea of high-vis workwear.

Simultaneous rallies swept every capital city and several regional centres, with workers from construction, healthcare, education, maritime and the public sector making strong showings.