AS Monaco's victory at Lorient means that OM must settle for fourth spot in Ligue 1 and qualification for the Europa League.
Despite the raucous home support at a packed Stade Velodrome, OM almost fell behind in the opening minutes as Steve Mandanda was rounded by Floyd Ayite, only for the forward's tame effort to keep the hosts on terms.
Leca, who replaced Paris-Saint Germain loanee Alphonse Areola in the Bastia starting lineup, then fell to the floor when a firecracker, thrown from the crowd, looked to have struck the goalkeeper. Replays later suggested that he was merely stunned by the missile, which landed nearby.
The start of the second half was delayed by several minutes after further missiles were thrown onto the pitch, with the referee taking both teams off the pitch and prompting Marseille players to plead with their own supporters to behave.
With Monaco's Champions League place already confirmed, the atmosphere at the Stade Velodrome became more jovial before loanee Lucas Ocampos scored another late on to round off a comprehensive victory.