The Marylebone Association, which previously campaigned against pedestrianising the street, said Mr Khan’s plans risked making the current situation worse.
“After dark, the street is very different to during the day, and having vehicles moving past makes it a lot more comfortable,” said a spokesman.
They noted that the longer walks pedestrians carrying shopping bags will face may deter shoppers from the area.
Steve McNamara, general secretary of the London Taxi Drivers’ Association, said black cab drivers had “lost confidence” in the police’s ability to keep Oxford Street safe.
“My members have lost all confidence in the police. Of course, that’s compounded by the level of street crime that they witness on a daily basis,” Mr McNamara said.
“The big thing that our members witness on a daily basis is phone snatches in London. And so they’re thinking, well, what’s the point in me ringing up and reporting that when nobody’s doing anything about it?”
Laura Vicinanza, a spokesman for Inclusion London, the disability campaign group, echoed concerns over the plans and warned they would make it harder for disabled people to reach the shops.
She said: “Fully pedestrianising Oxford Street would make it extremely difficult for disabled people, particularly people with mobility impairments to travel safely and independently.”