More than 40 people were infected with the new coronavirus after attending a mass in Frankfurt earlier this month, German media reported on Saturday.
Six people were admitted to hospital, local health authorities told the news agency DPA.
The service was held on May 10, a few days after the reopening of places of worship in Germany, the daily Frankfurter Rundschau reported.
Officials in the nearby Main-Kinzig-Kreis district had announced on Friday that 16 people who attended an event in Frankfurt became infected.
Germany began to relax the restrictions put in place to control the spread of the virus at the beginning of May, as it saw a strong decline in new cases.
But fears remain of a second wave in the country, which has been less affected than some of its European neighbours.
Earlier on Saturday, authorities in the northern state of Lower Saxony announced that seven people had been infected in a restaurant in the city of Leer.
A total of around 50 people have been quarantined.
The question of allowing religious services is particularly sensitive in Germany and beyond.
In France, a week-long gathering organised by an evangelical church at the end of February, attended by some 2,000 people, was the source of dozens of outbreaks across the country.
In the United States, President Donald Trump on Friday called on state governors to authorise the opening of places of worship, which he described as “essential” at a White House news conference.
Germany had 177,850 officially declared cases of the coronavirus as of Saturday, with 8,216 deaths, according to the Robert Koch Institute.
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