According to UEFA's Club Licensing Benchmarking Report, Barca's cumulative attendance in 2015-16 was 1.5million, besting United's 1.43m.
Bundesliga giants Borussia Dortmund outstrip both in terms of average attendance, with 81,178 at their Signal Iduna Park home, although Germany's top flight being an 18 as opposed to a 20-team competition contributed to their overall attendance coming in third at 1.38m.
All three sides enjoyed better attendances than Real Madrid, who attracted an average of 71,280 to the Santiago Bernabeu and 1.35m in a season that concluded with them being crowned European champions for an 11th time.
Dortmund's bitter domestic rivals Bayern Munich rounded out the top five with 75,000 fans on average and 1.275m visitors to the Allianz Arena overall.
Manchester City's redevelopment of their Etihad Stadium helped the Premier League side to record the biggest increase in average club attendance in Europe, with a rise to 54,041 from 45,365 in the 2014-15 campaign.
City's total attendance breached the one-million mark, leaving them eighth overall, one place above Newcastle United.
Newcastle's relegation from the Premier League last term is likely to further boost figures for the second-tier Championship, which was the fourth-best attended league in Europe behind the Premier League, Bundesliga and LaLiga, while sitting above Serie A.
In total, more than 170m fans went to European league matches in 2015-16, with 55 million in England and Germany alone. This marked an increase of 2.6m on last season as 14 leagues achieved their best attendance figures for more than a decade.
The Bundesliga, Premier League and La Liga are listed in the top 20 average attendances for all sporting events across the globe. The World Cup is football's highest entry in third, behind rugby union's 6 Nations and the NFL.