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England got their quest for a first major tournament title in 58 years off to a winning start as Jude Bellingham's header was enough to beat Serbia 1-0, while the Netherlands grabbed a late 2-1 win over Poland on Sunday.
Christian Eriksen enjoyed a fairytale return to the European Championship, three years after suffering a cardiac arrest on the pitch, by scoring the opening goal, but Denmark were held 1-1 by Slovenia.
England had been hyped as one of the favourites for the tournament as Gareth Southgate aims to finally steer a richly-talented generation to glory.
However, they left plenty of room for improvement in a flat tournament debut in Gelsenkirchen.
Bellingham was the exception as the Real Madrid midfielder confirmed his status as one of the world's rising stars with a man-of-the-match display.
"The first half showed why we can score goals against any team and the second half showed why we can keep clean sheets against any team," said Bellingham.
"I feel like in every game I can make an impact. I can decide a game. It's a release for me to play football, it's my favourite thing in the world, so I don't feel any pressure when I go out there."
The 20-year-old became the first European player to grace three major tournaments before turning 21.
And he capped a brilliant individual display with the only goal when he headed in Bukayo Saka's deflected cross on 13 minutes.
Harry Kane saw a late header turned onto the bar by Serbia goalkeeper Predrag Rajkovic.
But England were forced to see out a nervy finale as Jordan Pickford turned over Dusan Vlahovic's dipping drive before Kane headed away Veljko Birmancevic's goalbound shot.
The match had been declared as "high risk" by German police and there were minor clashes between rival fans before kick-off which saw seven Serbian fans taken into custody.
- Super sub Weghorst -
For the Netherlands, Wout Weghorst struck in the 83rd minute, just two minutes after coming on as a substitute, to secure an opening Group D victory against Poland, who were without the injured Robert Lewandowski.
"I am very happy. This is indescribable, especially at a final tournament. So special. This is personally a dream scenario," said Weghorst.
"The absolute goal is to win that cup together."
Dutch fans packed the streets of Hamburg, creating an orange wall of noise hours before kick-off.
It was the Poland supporters whose voices echoed around the Volksparkstadion when Adam Buksa headed their team in front early on, but Cody Gakpo levelled an entertaining encounter before the half-hour mark with the aid of a deflection.
Gakpo, Memphis Depay and Denzel Dumfries all went close as the missed chances piled up for the Oranje, and Ronald Koeman's men looked to be running out of ideas until Weghorst swooped to slot home a deserved winner.
The Dutch, the 1988 winners, are hoping to end a poor recent Euros record, having not even reached the quarter-finals since 2008.
Earlier, police shot and injured a man who threatened them with an axe and a Molotov cocktail in Hamburg before kick-off, although a police spokesman said there was no indication the incident was linked to the game.
- Eriksen delight -
Eriksen collapsed during Denmark's opening game at the delayed Euro 2020.
Exactly 1,100 days on, the Manchester United midfielder put the Danes in front with a sweet strike in Stuttgart.
"I do think this time my story at the Euros is very different from the last one," said Eriksen.
"Luckily it's been a lot of games since the last time and since it happened. I felt confident in playing again, so in that sense I was just happy to be back playing."
After the shock of the Eriksen incident, Denmark rode a wave of emotion three years ago to reach the semi-finals.
Yet, they disappointed in a group stage exit at the 2022 World Cup and failed to get off to a winning start as Erik Janza's deflected effort earned Slovenia earned a point.
Next up for the Danes is a rematch of their Euro 2022 semi-final against England, while Slovenia take on Serbia on Thursday.