Loud cries of "freedom" rang around the arena throughout the 90 minutes.
Two banners reading: "Freedom for political prisoners" and "Europe shame on you" were held up by fans behind one of the goals.
"People come to the Camp Nou to express themselves peacefully and support their team. We play for them," said Barcelona coach Ernesto Valverde. “The team are showing that what is happening with the politicians isn't a distraction."
Alcacer opened the scoring on his first league start since the first day of the season when Sevilla defender Sergio Escudero fluffed an interception mid way through the first half.
The ball fell invitingly to the striker to slot past Sevilla goalkeeper David Soria.
Fifteen minutes after the pause, the visitors were level through Guido Pizarro’s powerful header. Alcacer volleyed home Ivan Rakitic’s pass to claim his second.
"You have to take the chance given to you by the coach," the 24-year-old told BeIN Sports Spain. "More than my game you have to look at it from a team perspective and these games where you are made to suffer mean more."
Barcelona, under new boss Valverde, have won 10 and drawn one of their 11 games to boast 31 points
"We are top. We aren't dropping points and we are going into the international break happy," added Valverde.
"Afterwards we have a tough schedule. We have strong rivals behind us and we need to remain strong to stay on top."
Valencia are second on 27 points after sweeping past Leganes 3-0 to register a club record seventh straight league win.
Dani Parejo gave Valencia the lead from a free kick early in the first half. But the hosts had to wait until late to secure the three points.
Rodrigo headed in the second after 71 minutes and Santi Mina scored the third from the penalty spot eight minutes from time.
"No one will go undefeated through 38 games, but we are going to enjoy the moment," said Valencia coach Marcelino Garcia Toral.
"In the dressing room there is no euphoria, there is realism and constant analysis. We know how difficult it will be to keep this up, it's practically impossible."