What you need to know:
- United, West Ham and Arsenal players -- the Premier League teams in action in Europe's second and third tier tournaments -- all wore black armbands after the Queen died aged 96 earlier in the day
Manchester United led English football's tributes to Queen Elizabeth II before their 1-0 defeat against Real Sociedad in the Europa League, while Roma boss Jose Mourinho was in mourning after the monarch's death on Thursday.
United, West Ham and Arsenal players -- the Premier League teams in action in Europe's second and third tier tournaments -- all wore black armbands after the Queen died aged 96 earlier in the day.
There were sombre atmospheres at Old Trafford and the London Stadium, with United and West Ham both holding a minute's silence prior to kick-off.
West Ham fans sang 'God save the Queen' during their 3-1 win against Romanian side FCSB in the Europa Conference League.
The death of the nation's longest-reigning monarch was announced after Arsenal had already kicked off at FC Zurich, so the Gunners had a minute's silence before the start of the second half of their 2-1 Europa League win.
Former United, Chelsea and Tottenham manager Jose Mourinho paid a sombre tribute to the Queen after Roma's 2-1 Europa League loss at Bulgarian side Ludogorets.
"I'm so sorry. I have lived in England for many years, my family is there," Mourinho told Sky Sport Italy.
"I don't think there's anyone who doesn't appreciate this great woman, I'm sorry."
During an immaculately observed minute's silence in Manchester, United's players linked arms while Sociedad did the same on the other side of the centre-circle.
Several United fans were seen crying in the stands, while Sociedad's fans held their blue and white scarves aloft in tribute during the minute's silence.
There had been no pre-match music and the digital advertising hoardings were switched off, while flags at Old Trafford were at half-mast as a sign of the club's "utmost respect" for the Queen.
"Manchester United shares the sorrow of the entire nation following the announcement from Buckingham Palace on the passing of Her Majesty, the Queen," a club statement said.
"The club recognises her immense contribution to public life, including sport, both here in the United Kingdom, across the Commonwealth, and around the world."
UEFA President Aleksander Ceferin added: "UEFA and European football are truly saddened by the passing away of Her Majesty Queen Elizabeth II, one of the world's most-respected figures.
"Our thoughts are with her family and the President of the English Football Association Prince William, as well as with the citizens of the United Kingdom and Commonwealth realms."
The usual chants in support of United and against the club's owners the Glazer family were heard once the game started, but the atmosphere felt more subdued than usual.
Erik Ten Hag described Brazil forward Antony as United's "missing link" after the Brazil forward scored on his debut against Arsenal on Sunday following his £82 million move from Ajax.
But the 22-year-old struggled to make any impact during a disjointed United performance.
Cristiano Ronaldo was making only his second United start this season, while Brazil midfielder Casemiro was given his first start since arriving from Real Madrid.
Neither player could spark United, who saw a 35th minute header from Ronaldo correctly disallowed for offside.
Sociedad clinched their victory in the 59th minute when United defender Lisandro Martinez was controversially penalised for handball.
The ball only hit Martinez's arm after bouncing off his knee but following a VAR check the decision was confirmed and Brais Mendez stroked the spot-kick past David De Gea.
After winning four successive Premier League games, United were beaten for the third time in boss ten Hag's reign.
Brazilian teenager Marquinhos scored on his Arsenal debut as the Premier League leaders recovered from their first defeat of the season at United last weekend.
Marquinhos, signed from Sao Paulo in June, finished first-time from a low Eddie Nketiah cross in the 16th minute to put Arsenal ahead.
Zurich equalised just before half-time when Mirlind Kryeziu converted a penalty after a foul in the box by Nketiah.
Nketiah made amends by scoring the winning goal just after the hour mark, heading in from Marquinhos's cross.
Elsewhere in the Europa League, PSV Eindhoven and Bodo/Glimt drew 1-1, with Cody Gakpo after Albert Gronbaek had given the Norwegian champions the lead.
Lazio beat Feyenoord 4-2 thanks to Matias Vecino's double.