Vancouver police are investigating after a woman was sexually assaulted in a busy park near the Main Street SkyTrain terminal this weekend.

Investigators say the 43-year-old was walking through Thornton Park near Main Street and Terminal Avenue just after 8:30 p.m. on Saturday when she was attacked.

"The man forced her to the ground, punched her in the head, and then sexually assaulted her by forcefully groping her," Const. Lindsey Houghton told reporters at a police press conference Monday morning.

The victim fought off the man, who took off south through the park. The woman was taken to hospital and received four stitches on her forehead.

Houghton said police have dealt with minor incidents at the park but the assault is unusual because it happened during daylight hours.

"That's a very busy part of the city and Thornton Park is very well used," he said.

Police collected statements from several witnesses at the scene. Houghton said it's possible people saw the attack but didn't realize what was happening.

"These incidents can happen very quickly. In a matter of seconds this could have happened and the man could be off running."

The suspect is described as a scruffy looking white male between 30 and 40 years old, with a medium build and some facial hair. He is described as between 5-10 to 5-11 tall.

Houghton said the incident is not related to two assaults on Vancouver's west side within minutes of each other on Thursday last week.

Two women were pushed to the ground and repeatedly groped on Vine Street. The suspect is described as a white man, 5-7 tall with a stocky build.