The River Troupe Gospel, a volunteer group, rehearsed on 11 September ahead of an open-air performance two days later for a local festival in Sallent, a town in the province of Barcelona.
It was their first public show since the beginning of the pandemic.
One member of the chorus tested positive following the performance on 13 September, forcing more than 40 other members and their close contacts into isolation, the chorus said.
Since then, at least 30 singers have tested positive, the Sallent municipal government said.
Although the chorus claims it complied with most health safety measures, including temperature checks on arrival, hand washing, social distancing and masks on for most of the rehearsal, the venues windows were closed to avoid moths and mosquitoes.
The group said it had switched on air conditioning to fight the heat.
It is not known if any of the singers’ relatives have also been infected.
Experts have warned that in certain circumstances, such as during medical procedures or when people are in close indoor spaces while singing or shouting, coronavirus can spread in the air.
Choir practices in the US have been identified as a super spreading event in which hundreds of people later came down with the virus.
The Sallent town hall believes there was no risk of infection during the 13 September performance following the rehearsal because it was outdoors with required safety measures.
It comes as Spain continues to battle a second wave of the coronavirus.
The country has a confirmed death toll of over 31,000.
Additional reporting by Associated Press