Stephane Dujarric, the spokesman, conveyed Guterres' concerns in a statement over the increased tensions in Western Sahara as the rally participants prepare to cross the Guerguerat buffer zone on Monday.
Western Sahara is now mostly controlled by Morocco. But the Polisario Front has been campaigning for its independence.
In the statement, the UN chief called on all actors to the conflict to exercise maximum restraint and to defuse any tensions.
Media have reported threats from the Polisario Front to disrupt the race.
The reports said the group warned that it will respond to any actions that aim to "undermine the integrity of Western Sahara."
Sunday's UN statement said it is necessary to allow "for regular civilian and commercial traffic to proceed."
The UN chief also warned against any action that "may constitute a change to the status quo of the buffer strip," reiterating the UN's commitment to support the "parties to reach a just, lasting and mutually acceptable political solution to the conflict in Western Sahara in accordance with Security Council resolutions."