Porto extended their record-breaking unbeaten run in Portugal's Primeira Liga to 57 games on Sunday.
But their 1-0 win at Vitoria was overshadowed by a worrying incident in second-half stoppage time involving a pitch invader.
One rogue fan ran onto the field and aimed kicks at two Vitoria players.
First the pitch invader had a couple of digs at the right ankle of Vitoria captain Rochinha.
He then took a much bigger swing at winger Geny.
Neither player was injured but the incident raised serious security concerns.
Staff from both benches stormed onto the pitch in an effort to catch the violent fan, who was eventually taken away by security.
Porto defender Pepe was seen gesturing towards the pitch invader and security staff in an apparent effort to calm the situation.
Mehdi Taremi had earlier scored from the penalty spot to give Porto the lead, before Oscar Estupinan was sent off for the hosts for picking up two yellow cards.
Porto's win saw them end the weekend six points clear at the top of the table.