Sydney’s post-match jubilation was marred by violence last night after a man was brutally beaten while coming to the aid of two women following the Panthers-Bulldogs clash.

Police have praised the behaviour of the NRL preliminary final’s crowds, saying fans were overwhelmingly well behaved, apart from the isolated incident.

The Bulldogs will advance to the Grand Final to take on the South Sydney Rabbitohs next week after beating Penrith 18-12 last night.

Traditional Doggie heartland, Belmore, was awash with a sea of blue and white as thousands of fans took to the streets to celebrate.

Police have praised crowds, saying people were well behaved. (9NEWS) (9NEWS)

Police were ready for crowds, using general duties officers and the riot squad to keep fans moving.

No violence was reported, although huge crowds did cause minor traffic delays on Burwood Road and around the adjacent leagues club.

Happy fans took to the streets to celebrate the Bulldogs advancing to next week's Grand Final. (9NEWS) (9NEWS)

While things were mostly well behaved, police are investigating a violent bashing on a train bound for Sydney’s Central Station shortly before midnight.

Police said a 48-year-old man approached two younger men onboard in an "attempt to de-escalate matters" after they started arguing with two woman.

The pair attacked the older man, badly beating him as the train arrived at Central.

The man was hit repeatedly in the face by the pair, believed to be wearing Penrith Panthers gear.

Paramedics were called to the station and treated the man for serious facial injuries, including a broken nose.