Australian: Police locked down the center of the country's biggest city

(Reuters) - Australian police locked down the center of the country's biggest city on Monday after an armed assailant walked into a downtown Sydney cafe, took hostages and forced them to display an Islamic flag, igniting fears of a jihadist attack.

Police said they knew of one armed assailant involved in the incident at the Lindt chocolate cafe in the heart of Sydney's financial district, but there could be more.

Police, including heavily armed paramilitary officers, cordoned off several blocks around the cafe as negotiators tried to defuse one of the biggest security scares in