The court heard that del Castillo had been with Garcia at his Lima home, the night Fujimori sent tanks to shut down Peru's judiciary and legislature in a power grab.
He said soldiers arrived to arrest Garcia but found del Castillo who was stalling in order that Garcia could flee.
Del Castillo said that soldiers opened fire on the house.
"We heard on a loudspeaker: 'In the name of the Joint Command of the Armed Forces, come out with your hands up, Dr Garcia, and everyone else inside.' Then they started to fire on the house."
Del Castillo said that they wanted to "physically eliminate him [Garcia], there's no doubt".
Garcia escaped and later took asylum in Colombia and then France.
Del Castillo was a legislator for Garcia's Aprista party at the time.
Prosecutors in the trail are attempting to show Fujimori's knowledge about the arrests.
Del Castillo referred to a document signed by General Nicolas Hermoza, the highest-ranking military commander at the time, which stated that the arrests were carried out "on orders from above".
Hermoza verified the authenticity of the document in a trial in 2005 in which he was sentenced to eight years in prison for corruption.
"It was a kidnapping. They never told me why I was being arrested over those five days I was held in military installations," said del Castillo.
Fujimori argued that the actions were needed to counter the out-of-control violence by the Shining Path and Tupac Amaru Revolutionary Movement leftist fighters.
The group was virtually eliminated during Fujimori's presidency.
Only a small number of their fighters are still active in the Peruvian jungle.
Fujimori is on trial for kidnapping and allegedly sanctioning two army death-squad massacres in 1991 and 1992 which killed 25 people.