Hurtado said she had not ordered the surveillance
The head of Colombia's intelligence services has resigned after admitting her agents spied on left-wing political opponents of President Alvaro Uribe.
Maria del Pilar Hurtado said she was stepping down to preserve the honour of the security services, known as Das.
Two days ago, an opposition senator, Gustavo Petro, accused them of monitoring his movements.
Ms Hurtado's predecessor, Jorge Noguera, is being probed over alleged links to right-wing paramilitaries.
Mr Noguera was removed from his office in 2005.
Mr Uribe has named Joaquin Polo, the current deputy-head of the Administrative Security Department (Das), as Ms Hurtado's replacement.
Senator Petro is known for his investigation into alleged links between right-wing groups and allies of the president.
He said that members of his Democratic Pole party had also been under surveillance.
Memos have emerged which spoke of monitoring Petro's "contacts with people who offer to testify against the government".
Ms Hurtado said that neither she nor President Uribe had ordered the surveillance.
"At no time did I receive or give any instructions linked to the incidents that have been made public," she said.
"The country still can and should count on the Das; it would not be fair for the work of hundreds of agents to be stained by the actions of a few," Mr Hurtado added.