About 50 people have been caught up in a large brawl that spilled on to a main road and stopped peak-hour traffic in Sydney's west.

Police are still investigating what sparked the brawl on O'Connell Street in Parramatta about 5.15pm on Tuesday.

Footage taken from an office block overlooking the busy street shows dozens of people swarming on to the road, where a number of them begin trading blows.

"They're shaping up," a male witness says in the footage, supplied to Channel Seven.