At least 74 people have been killed in clashes between rival fans following a football match in the Egyptian city of Port Said.
Scores were injured as fans - reportedly armed with knives - invaded the pitch after a match between top-tier clubs al-Masry and al-Ahly.
Officials fear the death toll could rise further.
It is the biggest disaster in the country's football history, said the Egyptian deputy health minister.
"This is unfortunate and deeply saddening," Hesham Sheiha told state television.
Some of the dead were security officers, the Associated Press news agency quoted a morgue official as saying.
Field Marshal Hussein Tantawi, the head of Egypt's ruling army council, went to a airbase near Cairo to welcome back al-Ahly players who were flown back from Port Said on a military aircraft.
"This will not bring Egypt down... These incidents happen anywhere in the world. We will not let those behind it go," he said, AP reports.
A statement posted on the Supreme Council of the Armed Forces' Facebook page announced three days of national mourning, beginning on Thursday.
The statement also promised a full investigation into the incident.
The BBC's Jon Leyne in Cairo says it appears some fans had taken knives into the stadium.
Our correspondent says the lack of the usual level of security in the stadium might have contributed to the clashes.
Police in Egypt have been keeping a much lower profile since last year's popular protests that ousted President Hosni Mubarak from power.
Egyptian fans are notoriously violent, says our correspondent, particularly supporters of al-Ahly known as the Ultras.
They have been heavily implicated in confronting the police during recent political protests, our correspondent adds. There is speculation that the security forces may have had an interest in taking on al-Ahly supporters.
Wednesday's violence broke out at the end of the match, which, unusually, Port Said side al-Masry won 3-1.
Witnesses said the atmosphere had been tense throughout the match - since an al-Ahly fan raised a banner insulting supporters of the home team.
As the match ended, their fans flooded onto the pitch attacking Ahly players and fans.
A small group of riot police tried to protect the players, but were overwhelmed.
Part of the stadium was set on fire.
'Rage in their eyes'
Officials say most of the deaths were caused by concussions, deep cuts to the heads and suffocation from the stampede.
"This is not football. This is a war and people are dying in front of us," al-Ahly player Mohamed Abo Treika said.
Hani Seddik, who played for al-Ahly as a teenager, told the BBC: "I don't think this is about football. These trouble-makers were not football fans."
"How were they allowed to carry knives into the ground? To me, this is the actions of people who do not want the country to be stable and want to put off tourists from coming here," said Mr Seddik, who was watching the match on TV in Cairo.
One al-Ahly fan in Cairo told the BBC that a large march from al-Ahly club to the Interior Ministry is being planned for tomorrow.
"People are angry at the regime more than anything else... People are really angry, you could see the rage in their eyes," Mohammed Abdel Hamid said.
Meanwhile, the Muslim Brotherhood - which has emerged as Egypt's biggest party in recent elections - blamed supporters of ousted President Hosni Mubarak for the violence.
"The events in Port Said are planned and are a message from the remnants of the former regime," Muslim Brotherhood lawmaker Essam al-Erian said.
He went on by saying that the army and police wanted to silence critics demanding an end to state of emergency in the country.
In Cairo, another match was halted by the referee after news of the Port Said violence. It prompted fans to set parts of the stadium on fire.
All premier-league matches have been cancelled and the newly-elected Egyptian parliament is to hold an emergency session on Thursday.
Fifa President Sepp Blatter issued a statement, expressing his shock over the incident.
"This is a black day for football. Such a catastrophic situation is unimaginable and should not happen," he said.