Cerpa was born in Arequipa. Originally a union leader, in the early 1980s he became involved with MRTA, apparently disappointed with the reversal of the policies of Juan Velasco. He quickly rose to be the leader of the San Martín Zone Committee and led and/or participated in:
Putting a red communist flag in the "Plaza Unión" on September 9, 1984.
Assault and armed robbery against Electro Perú, Lima, on May 25, 1985.
Cerpa was the leader of the MRTA commando squad that seized the Japanese ambassador's residence in Lima in December, 1996, which started what is now called the Japanese embassy hostage crisis. The MRTA's main demand was for the exchange of the hostages for 465 members of the MRTA in prison, including Cerpa's wife Nancy Gilvonio, the Chilean members of the organization, and U.S. citizen Lori Berenson. The government rejected the demands, and on April 22, 1997, after 126 days, a 140-man team of commandos of the Peruvian Armed Forces stormed the Japanese ambassador's residence to rescue all the hostages. One hostage and two soldiers died in the assault, as did all fourteen (MRTA) rebels.