“All conditions are set. We need to unite all Colombians and put an end to the conflict,” said Mr Santos at a march in the capital, Bogota.
The talks, aimed at ending five decades of violence, are taking place in Cuba.
The demonstrations have attracted large crowds in Cali, Barranquilla, Santander and other Colombian cities.
For more on this story, visit: BBC News – Colombians march in support of Farc peace talks.