The five-time Ballon d'Or winner converted a penalty in the 72nd minute, after a Ukraine player was judged to have handled a long-range shot in the box.
Messi opened the scoring with a pinpoint effort across Gianluigi Buffon on the stroke of half-time, while Ivan Rakitic finished from closerange after Messi's low cross was diverted away from goal on 56 minutes.
Tyrick Mitchell tapped in from closerange after a fine cross by Adam Wharton in the 58th minute and French youngster Olise capitalised on a Casemiro mistake to power a strike beyond a wanting Onana.
Pep Guardiola's side, taking part in the tournament after winning last season's Champions League, made the ideal start as Alvarez struck after 40 seconds, chesting in from closerange after Nathan Ake's long-range strike hit the post.
An eyewitness described the boy as having been shot at closerange, despite not being involved in the demonstrations. "This boy was not part of the protesters.
The 20-year-old Isaac smashed the ball into the roof of the net from closerange to hand fourth-tier Italian club Folgore Caratese a 1-0 win over Caronnese.
The former Manchester City and Liverpool striker netted from the penalty spot after just six minutes and tapped in his second from closerange. In between, Alassane Plea broke clear and slotted in for the home side.
One of the boys, Endurance, tried to run out of fear but was shot twice at closerange," Iyaragba said. "This prompted the infuriated youths of the community to go on a rampage, destroying company vehicles and property.
Nathan Ake had an early close-range header saved by Arsenal keeper David Raya before going off injured while former City striker Gabriel Jesus posed an occasional threat, just failing to connect with Bukayo Saka's cross inches from goal after
The 40-year-old tucked in from closerange inside two minutes at Stadio Pierluigi Penzo on Sunday. His goal against Venezia was his eighth Serie A goal of the season and his first of 2022.