Two Benfica players have been taken to hospital after their team bus was attacked following their first behind-closed-doors match.
German Julian Weigl and Serbian Andrija Zivkovic were hit with shrapnel and taken to hospital as a precaution.
Portuguese champions Benfica missed the chance to move clear of title rivals Porto at the top of the table after being held 0-0 by Tondela.
Fans had gathered to support the team outside Benfica's Estadio da Luz.
The attack occurred on a major motorway between the stadium and training ground.
"Benfica denounce and lament the criminal stoning the bus of its players was victim to," a Benfica club statement read.
"We guarantee total collaboration with authorities to whom we appeal for the greatest effort to be made in identifying the delinquents responsible for these criminal acts.
" The top two in the primeira liga table are now level on 60 points with nine rounds of matches remaining.