An international student group started a campaign that urges shoppers to avoid Gap clothing stores this holiday season by placing ads Thursday in newspapers at several leading universities.

The United Students Against Sweatshops said it is acting in response to anti-union activity at factories manufacturing Gap products in El Salvador and South Africa.

Ben McKean, the group’s spokesman, said Gap suppliers in those two countries have used physical harassment, termination and blacklisting to impede organizing efforts.

He called the group’s action a two-month “holiday campaign” meant to convey a message from the foreign factory workers to the American public. McKean said the effort is not a boycott.

The organization will assess the effectiveness of the campaign early in the new year to determine if it will continue, McKean said.

A Gap spokeswoman declined comment, referring to a section of the company Web site on “ethical sourcing,” where the retailer pledges to promote fair treatment for workers by the vendors who make Gap clothing.