The masked gang burst into the Grand Hyatt hotel in central Berlin, where the tournament was taking place, threatening security staff and prompting a brief panic, police spokeswoman Heidi Vogt told AFP.
Berlin's Tageszeitung newspaper reported on its website that six robbers armed with Kalashnikov assault rifles and grenades made off with €800,000 ($1.1 million).
"Several masked, armed individuals entered the Grand Hyatt and fled with a haul of money," Vogt said, without giving details of the sum involved.
She said a number of the tournament's players were injured in the incident, though none were seriously hurt.
Four of the attackers entered the hotel from Potsdamer Platz, one of the German capital's most important and popular squares, while two others kept watch, the Tageszeitung reported.
A security guard briefly stopped one of the robbers, who hit him over the head with a weapon and fled, the newspaper said.
Nearly 1,000 players are to take part in the five-day poker tournament, the first to be held in Berlin by the European Poker Tour. German tennis legend Boris Becker took part in the tournament's launch night.