Police say they are taking "extremely seriously" a witness report of a child being taken into a van before it drove off.
A boy of around six or seven was seen being taken into the side of a black Volkswagen transporter with an 02 licence plate in Redhill at 4.43pm on Thursday.
A Surrey Police spokesman said: "The suggestion is that they have gone into the side of the van."
Police said the van was a model with a sliding side door and no rear windows.
A mountain bike believed to belong to the child was left behind at the scene.
The boy is described as white and around 4ft, wearing a red T-shirt and navy jeans.
Superintendent Bex Smith said: "This is an incident we are taking extremely seriously and are asking for anyone with any information regarding the child, the van or what happened to come forward.
"If you were in the area, perhaps commuting to or from Redhill train station at the time please get in touch with us if you saw anything suspicious.
"At this stage we are not ruling out a number of options, including whether the child and the person in the van know each other."
A National Police Air Service helicopter is assisting in the search.
The van, which bore no logo, is described as having three alloy wheels,, with one front nearside missing. The alloys were a BBS make.
It had chrome sidesteps.
Anyone with any information should contact Surrey Police by calling 999 in an emergency or 101, or report it online at www.surrey.police.uk quoting Operation Bigfoot.
Alternatively contact the independent charity Crimestoppers anonymously on 0800 555 111.