Barca had impressively beaten Juventus, Manchester United and Real Madrid in the United States last month but looked to be on course for its first defeat of pre-season until Alcacer's impressive intervention after 79 minutes.

Playing in its first match since the world-record sale of Neymar to Paris Saint-Germain, Barca managr Ernesto Valverde named a side mainly filled with fringe players, with Andres Iniesta, Lionel Messi, Gerard Pique and Luis Suarez among the stars missing out.

Host Nastic, which plays in the Spanish second tier, took a shock lead after 11 minutes when Manu Barreiro shot past Jasper Cillessen from a yard out after being set up by Omar Perdomo from the left.

Vermaelen, who spent last season on loan at Roma, limped off with injury after the goal, Marlon Santos replacing him.

Alcacer and Carles Alena went close for Barca either side of Valverde bringing on Munir El Haddadi and Marc-Andre ter Stegen at the break, while Gerard Deulofeu, Andre Gomes, Jordi Alba and Douglas entered the fray midway through the second half.

After Deulofeu came close to an equaliser it looked like the visitor was heading for a shock defeat, before Alcacer produced a moment of magic to convert his set piece from almost 30 yards.

Barca is back in action against Chapecoense in the Gamper trophy on Tuesday, before its competitive season gets under way against Real Madrid in the first leg of the Supercopa de Espana on 13 August.