But Sussex Police said there have been no reports of missing children.
The force said they were looking into whether the child was actually with their parents or another family member.
Sussex Police also said it was possible the report was an 'innocent mistake in good faith' in a tweet that has now been deleted.
The force released CCTV images of a man with a child and urged anyone who recognised them to get in touch.
The man is seen bending down beside the girl in one image and they are seen walking side by side in the other.
Superintendent James Collis said: "From all our enquiries we think the man and child in this image may be the ones our original caller is referring to.
"Active enquiries are continuing but we are also considering the possibility that the man and child are family members or at least known to each other, and that the caller may have misinterpreted what they saw but was nevertheless acting in perfectly good faith.
"If you are that man, or a family member or friend who has information about this incident, please get in touch with us as soon as possible by ringing 101 quoting Operation Boxworth, so that we can meet you and confirm that all is well with you and the child.
"And if anyone else recognises these people we also ask them to call us right away."
An alert went out to hundreds of taxi drivers that a man aged between 70 and 75 may be involved in the incident, the Brighton Argus reports.
A spokesman from Streamline Taxis told the paper: "We received a message from police at 5.15pm about an abduction of a girl at Churchill Square.
"We were asked to put the message to our drivers. We haven't heard any more updates yet."