The singer was nearly run off the road by two drivers just hours after appearing on David Letterman's chat show, according to a report today.
A source told the Daily Star: "Madonna's driver noticed there were two big vehicles driving closely behind her and tried to ditch them by turning on to Fifth Avenue. He was particularly concerned as Madonna has recently been sent some disturbing mail - which has been somewhat threatening to say the least.
"But as he turned to go along Fifth Avenue, the two following cars tried to ram Madonna's car off the road and she was forced to dial 911. Eventually, after a high-speed chase, her driver managed to lose them and they reported it to police in a lengthy phone call. Madonna is carrying on as normal but has stepped up security."