At least six protesters have chained themselves to century-old trees that are due to be chopped down to make way for Sydney's new light rail line on the edge of Centennial Park.

Some 30 contract workers arrived on Thursday night, ready to start felling 50 trees that line Anzac Parade and Alison Road to make way for the $2.1 billion project that would connect Randwick to Kensington.

Workers set up barriers around old trees due to be cut down to make way for Sydney's new Light Rail. Credit:Wolter Peeters

But a group of residents and environmental activists beat the high-vis army to it, lashing themselves to trunks and hoisting others into the branches in a last-ditch attempt to save the trees.

"I'm prepared to stay here all night, if I have to," said Louise Boronyak-Vasco as she stood chained to a tree on Alison Road.