Police have arrested a 21-year-old man over the alleged sexual assault of a woman at knifepoint in a park in Melbourne's north on Saturday night.

Key points: A man of no fixed address was arrested at Preston Police Station this morning, police said

A man of no fixed address was arrested at Preston Police Station this morning, police said Detectives had circulated an image based on the description of the alleged attacker

Detectives had circulated an image based on the description of the alleged attacker The woman seized her attacker's knife and stabbed him during the assault, police said on Sunday

Yesterday, police had appealed for the public's help in finding a man who allegedly produced a knife as he attacked a 32-year-old woman in Thornbury's Mayer Park.

In a statement today, Victoria Police said the 21-year-old man, who was of no fixed address, attended Preston Police Station about 11:00am.

The man remained in custody and was assisting police with inquiries, police said.

On Sunday, police said the woman, who was walking a dog, had entered Mayer Park about 10:30pm on Saturday night after she noticed a man ahead of her acting suspiciously.

Police said the man approached her and asked for help to find a lost possession before he pulled out a knife, pushed her to the ground and sexually assaulted her.

The woman managed to grab the knife in the struggle that followed and stabbed him, according to police.