Palestine conceded a goal after just 64 seconds in a 4-1 loss to Iran at the Asian Cup on Sunday, while tournament favourites Japan survived a scare before beating Vietnam 4-2.

Hong Kong's first Asian Cup match in more than 55 years ended in a 3-1 defeat to the United Arab Emirates.

Palestine's opening match of the tournament coincided with the 100th day of the Israel-Hamas war, the grim milestone adding extra significance to an occasion that went beyond football.

Palestinian flags were scattered among the crowd of nearly 28,000 at Education City Stadium, and their goal just before half-time got the biggest cheer of the night.

Loud cries went up for the Palestinian anthem before the game and the team's players put their arms around each other's shoulders.

A brief moment's silence was held before kick-off, the quiet punctuated by cries of "free Palestine".

But Palestine were behind after little over one minute, attacker Karim Ansarifard rattling the ball into the bottom corner for an Iran team who are among the favourites in Qatar.

Iran, who are pursuing a record-equalling fourth continental crown, made it 2-0 on 12 minutes when Shojae Khalilzadeh swept home unmarked from close range following a free kick.

Iran looked like scoring every time they went forward and they made it 3-0 seven minutes before half-time, Feyenoord winger Alireza Jahanbakhsh playing in Mehdi Ghayedi to side-foot into the bottom corner.

But the biggest roar of the night came right on the stroke of half-time when Tamer Seyam headed in from close range, the Palestinian pointing skyward in a muted celebration.

Iran scored a fourth soon after the break through Roma's Sardar Azmoun.

Japan are looking for a record-extending fifth Asian title and looked set for a comfortable start to their campaign when Takumi Minamino gave them an 11th-minute lead against Vietnam.

But Vietnam, coached by Japan's 2000 Asian Cup-winning boss Philippe Troussier, turned the game on its head with two goals midway through the first half.

Japan came roaring back before half-time, Minamino scoring a second and Keito Nakamura curling home a sublime effort to send them into the break with the lead.

The second half was a far more sedate affair until Japan substitute Ayase Ueda made the win safe with a fourth goal in the 86th minute.

"I've been playing against Vietnam since I was young so I know how good a team they can be," said former Liverpool forward Minamino.

"But it wasn't just in defence -- they were really good in attack as well. I was surprised by how good they were."

Hong Kong were on the wrong end of three big VAR calls in their defeat to the UAE.

Two of the UAE's goals came from VAR-ruled penalties, while a late Hong Kong goal was chalked off after the referee checked the screen.

Coach Jorn Andersen warned his players before the game that the "gap is very big" between themselves and the continent's footballing elite.

But there was no yawning gulf in class as the tournament's lowest-ranked side kicked off their first Asian Cup campaign since 1968.