The Mean Girls actress was staying in the Chiltern Firehouse hotel while she rehearsed for the play Speed-The-Plow and she became concerned for her safety when a man camped outside the building to catch a glimpse of her.
According to the Sunday People newspaper, the admirer sent marriage applications to the accompanying restaurant to get Lohan to sign.
A source told the publication: "Lindsay's really shook up about it all. This man just wouldn't leave her alone. He knew where she was staying and was always there, waiting. She's been really scared."
Staff at the hotel called cops over the incident, and a spokesperson for the Metropolitan Police said: "On 17 July, police received an allegation of harassment. There have been no arrests."
Lohan has since moved out of the hotel to a luxury apartment in the Mayfair neighbourhood.