Pedro opened the scoring after Xavi intercepted Jesus Rueda's pass out of defence and former Barca trainee Marc Valiente's clumsy own goal put the champion two goals to the good at half-time.
Victor Perez's late penalty, after Martin Montoya fouled Manucho, denied Victor Valdes a clean sheet on his 500th appearance for the club but could not dampen the party atmosphere.
Captain Carles Puyol accepted the trophy but touchingly handed it to coach Tito Vilanova and defender Eric Abidal, both of whom have had treatment for cancer, to lift.
Barca remains on course for a club record 100 points should it win its remaining two games, away to city rival Espanyol and at home to Malaga.
At the other end of the table, Deportivo La Coruna beat Espanyol 2-0 to move out of the relegation zone above Real Zaragoza, which lost 2-1 at home to Athletic Bilbao.
Depor took the lead through a wonderful goal by Bruno Gama five minutes into the second half, with Nelson Oliveira making sure of the points with his fine lob in stoppage-time.
Real Zaragoza led through Helder Postiga's 15th-minute header but was pegged back in the final 10 minutes, Fernando Llorente with a scrappy equaliser for Athletic Bilbao before Ibai Gomez backheeled a 92nd-minute winner.
Rayo Vallecano survived a late fightback from Levante to take all three points at the Ciutat Valencia stadium with a 3-2 win and move up to eighth in the table.
Top scorer Piti gave Paco Jemez's side a first-half lead which was cancelled out by Robert Acquafresca but then quickly restored by Franco Vazquez.
Piti struck again, this time from the penalty spot, to increase Rayo's advantage but Vicente Iborra set up an exciting finish to the game by scoring with four minutes remaining.