Egypt has suspended its football league and ordered an investigation after at least 40 people were killed and dozens injured in a stampede and clashes between police and supporters of Zamalek football club at a game in Cairo, officials said.

According to witness accounts, violence erupted when police tried to set up barricades and used tear gas to disperse football fans trying to force their way into the army-owned stadium in the city's northeast on Sunday.

Egypt's public prosecutor ordered that an investigation begin immediately, while the incident prompted the government to postpone the Egyptian Premier League indefinitely, the prime minister's office said in a statement.

The Egyptian Interior Ministry said the clashes occurred after supporters of Zamalek, known as Ultras White Knights, tried to attend the game without buying tickets.

"Huge numbers of Zamalek club fans came to Air Defence Stadium to attend the match ... and tried to storm the stadium gates by force, which prompted the troops to prevent them from continuing the assault," the ministry said.

The fans posted on their group's official Facebook page that the violence began because authorities only opened one narrow, barbed-wire door to let them in. They said that sparked pushing and shoving that later saw police officers fire tear gas and birdshot.

"Suddenly they closed the gate and told us to get out through another gate," a witness told Al Jazeera.

"The police were in front and behind the gate. They fired tear gas. This caused panic and people fell on top of each other. We started to leave quickly. There were old people in the crowd and they were crushed by other fans."

Politicised fans

Relations between security forces and fan groups known as Ultras have been tense since the 2011 popular uprising, when football supporters played a key role in ending the rule of Hosni Mubarak.

Ultras have often clashed with security forces in and outside of stadiums, and as well as in the context of political protests, with police accused of using excessive force while confronting them.

Following riots in Port Said in 2012, when 72 people were killed in fighting between supporters, the number of people allowed to attend matches has been curbed and supporters have often tried to storm stadiums they are banned from entering.

One of the Zamalek fans said the supporters told security forces they wanted to enter the stadium to watch Sunday's game, but they were rejected.

"A few people walked in slowly and then we had someone telling us they would let us in from the back door," the fan said. "So people thought it would be OK, but then they started using tear gas and because of the large numbers [of people], some started to run and to jump over the fence [to escape the overcrowding].

"This is when they were confronted by the security forces," he added.

The fans accused security forces of a "massacre", but police denied using violence to try and pacify the crowd.

Many of the dead appeared to have died of suffocation after a stampede erupted.

Of those injured, many suffered broken bones and bruising, the Health Ministry said according to state news agency MENA.

Abdul Musa, a football journalist based in London, described the Ultras as some of the "fiercest fans in Africa". He said they had been threatening to cause violence at recent events.

"That's why there was a lot of police force today in Cairo," he told Al Jazeera.

The match continued despite the violence, provoking further outrage among the fans.

The decision to suspend the league season indefinitely is the same measure taken after the Port Said match three years ago where the home Al-Masry fans attacked Cairo's Al-Ahly supporters with explosives, knives and broken glass.

After that incident, the interim Egyptian government banned domestic league football for two years.

Last month, Al-Ahly and Al-Masry faced each other in the first game since the 2012 riot.