In one letter, a campaigner brands the decision to honour the former First Lady of the United States of America to "honouring Adolf Hitler for ridding Germany of Jews".
The campaign was started by the London-based Society for the Protection of Unborn Children (SPUC).
SPUC's director, John Smeaton, called on members to write to the university's principal, Dr Louise Richardson, damming their decision to honour Clinton.
He said: "Mrs Clinton's promotion of abortion is contrary to education, human rights, democracy, civil society and promoting opportunities for females around the world."
The visit is on September 13 and 14 to mark the university's 600th anniversary.
A spokesman for St Andrews said: "As a university, we stand for freedom of thought, freedom of expression and a willingness to listen to and learn from the views of all."