Mexico's Interior Department says the federal intelligence agency sent a plainclothes agent to tail an opposition presidential candidate, even though the candidate never asked for and apparently did not want a tail.

There have long been fears the ruling party was using the National Security and Intelligence Center for domestic spying. But few suspected the monitoring would be so blatant.

Interior Secretary Alfonso Navarrete told local media late Tuesday that a man caught on tape following presidential candidate Ricardo Anaya is indeed a longtime agent of the agency, known as CISEN.

Anaya posted a video of himself confronting the agent, who identified himself as a CISEN employee when asked why he had been following the candidate in a SUV.