Some 15,000 Sudanese police are trying to prevent rival fans clashing
Sudanese security forces have thrown a tight cordon around the capital Khartoum for Wednesday's Egypt-Algeria World Cup play-off.
Gates to Al-Merreikh stadium opened five hours before the 1730 GMT kick-off, with fans carefully segregated inside and outside the ground.
The winner of the game gets the last African place at the World Cup finals.
Sudan has deployed 15,000 police after a series of violent incidents involving fans of the two countries.
The play-off was arranged by Fifa at a neutral venue after Algeria controversially lost 2-0 in Cairo on Saturday, hours after their team bus was attacked by Egyptian fans.
Egypt's win left the top of Group C deadlocked, with both sides having identical records, but Algeria's coach blamed the defeat on injuries suffered by three of his players in the bus attack.
Violence between fans also flared after the game. At least 32 people were hurt, and the next day Egyptian businesses in Algiers were ransacked.
On Tuesday, the head of the Algerian football federation, Mohammed Raouraoua, said his Egyptian counterpart, Samir Zaher, was to blame for the trouble.
"He is the origin of all the events that occurred, including the barbaric aggression that injured... our players," Mr Raouraoua said.
But Egyptian Foreign Minister Ahmed Abul Gheit in turn told Algeria it must confront what he called the "saboteurs" who have attacked Egyptian businesses in Algiers.
Tickets for the match were reported on Wednesday morning to be selling on the black market for five times their original price.
Egypt and Algeria have been allocated just 9,000 tickets each, with capacity at the Al-Merreikh stadium reduced by 6,000 to 35,000 for security reasons.
Egyptian fan Nedal Nabil, who flew in from Dubai, told Reuters news agency: "There's going to be trouble because there aren't enough seats."
Hotels in Khartoum were booked out well ahead of the game, and the authorities have set up two camp sites for rival fans several miles apart.
The stakes for each country are high. The last time Egypt reached the World Cup finals was 1990, while Algeria's last appearance in the finals was in 1986.
There is a history of trouble between supporters of the two teams and riots broke out in Egypt in 1989 after an Egyptian win in Cairo.
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