The foot also appears to belong to a female, as the toenails were painted yellow, police said, but until pathology tests are done investigators cannot say if both the head and the foot are from the same person.
"Without a cause of death we can't call it homicide, but certainly foul play — there's definitely something amiss," said Peel Regional Police Acting Insp. Randy Cowan.
The size of the body parts do not suggest they are from a child, Cowan said.
A group of hikers found the bare right foot severed from the ankle down Wednesday in the Credit River near Hewick Meadows Park, a popular spot for family walks in the quiet residential neighbourhood. The police marine unit found the head Thursday.
"It's a grim discovery any time we get these calls," Cowan said. "It's certainly disturbing."
Search crews, divers and dogs combed the area, and police expected to be on scene for at least two days. Investigators are also asking for the public's assistance.
"We're looking for anybody that has seen anything suspicious within the last couple of weeks," Cowan said.
"We're also looking for people to be vigilant in the area if they're out walking and they happen to find something that causes them concern to give us a call."
Officers are sifting through missing person reports, in case any have a link to the body parts, Cowan said.
"Typically in the region there are always a number of outstanding missing people," he said. "The majority of those return safely...We are going through those reports right now, reaching out to family members."
Investigators believe the body parts were severed in the last few weeks.
Police couldn't say if the body parts were placed there or if they drifted there from another point upstream in the river, which flows from headwaters above the Niagara Escarpment to Lake Ontario.
Angela Elammar, who lives with her family in a home facing the park said she hopes no one was killed there.
"I don't want to be afraid here," she said.
The discovery left her thinking of her children, Elammar said.
"It scared me a bit because my kids take our dogs walking there," she said, adding they won't be walking the dog in the park again "for a long time."
Resident Sherine Ankle gasped in shock when she heard about the body parts. The park is where she takes her seven-year-old daughter to get some morning exercise, she said.
"It's upsetting, it's very upsetting," Ankle said. "It's sad we don't know who it is. It could have been us."