The 74-year-old was taken to the clinic Friday night amid tight security after coming down with stomach pains and high blood pressure, Alejandro Aguinaga, a legislator and head of Fujimori's medical team said.
"Ex-president Fujimori has digestive problems and for some months has been losing weight, so it was decided to do an endoscopy and various gastric tests," Aguinaga told reporters.
Fujimori was president of Peru from 1990 to 2000 when he was impeached on corruption charges.
Sentenced to 25 years in prison, Fujimori and his family say he is suffering from cancer of the tongue and have asked that his sentenced be commuted for humanitarian reasons.