More than 4,000 officers will be on duty during Thatcher's funeral in an attempt to guard against violent demonstrations.
In a brief statement, Chief Superintendent Julia Pendry, in charge of policing the London Marathon, said: "A security plan is in place. We will be reviewing our security arrangements in partnership with the London Marathon."
An estimated 500,000 spectators will line the streets to watch Sunday's race.
Zhang Xia, who is in her 30s and lives in London, told China Daily, "I watched the London Marathon last year with my daughter and husband. It's a fantastic and encouraging event, attracting many spectators. "We are considering whether we should watch it again this Sunday, after the blasts in Boston."
London Marathon chief executive Nick Bitel said: "Our security plan is developed jointly with the Metropolitan Police, and we were in contact with them as soon as we heard the news."
London Mayor Boris Johnson said: "The bombings in Boston are shocking, cowardly and horrific, and the thoughts of all Londoners this morning will be with the victims. Boston is a proud city built on history, tradition and a real sense of community."