Garcilaso went two points clear of Sporting Cristal - who were held to a 0-0 draw on the road at Alianza Lima - after their resounding win at the Estadio Municipal de Urcos.
Alfredo Ramua and Mauricio Montes were both on the scoresheet inside five minutes, as the hosts took control against the Group A cellar-dwellers in the second phase of the Peruvian top flight.
Montes made it a brace on 24 minutes, before Edwin Retamoso put the finishing touches on the result with an 80th-minute strike.
Sporting Cristal were unable to score at Alianza Lima on Sunday, leaving them two points in arrears of Garcilaso with four rounds of the season to play.
Lima sit in third on 64 points, two shy of Cristal (66) and four behind Garcilaso (68).
Fourth-placed Cesar Vallejo fell to their first loss in four, as they were trumped at home by Melgar 2-0.
Melgar, who are well out of title contention in sixth spot, are now unbeaten in seven matches.
Pacifico, seventh in Group A, had a 1-0 home win over fifth-placed Sport Huancayo at home.
In Group B, runaway leaders Universitario almost secured their spot in the league final as they came from behind to trump Juan Aurich 4-2 on the road.
Universitario trailed heading towards half-time courtesy of an early Edgar Balbuena penalty but drew level courtesy of a Rafael Guarderas goal.
The floodgates opened after the interval for Angel Comizzo's men, as Sebastian Fernandez, Manuel Ruidiaz and Diego Guastavino all scored to put Universitario 4-1 to the good before Daniel Arismendi's consolation strike late.
In other Group B results, Inti Gas had a 3-1 home win over Universidad San Martin, UTC Cajamarca got past 10-man Cienciano 1-0 and Leon de Huanuco were 4-2 winners over Jose Galvez.
