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 champions 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 remain on course for a club-record 100 points should they win their remaining two games, away to city rivals 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, who 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.
Zaragoza led through Helder Postiga's 15th-minute header but were pegged back in the final 10 minutes, Fernando Llorente with a scrappy equaliser 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 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.