A police officer was one of those killed in the explosion which occurred at around 9pm (1800 GMT) on Sunday, according to the Turkish news channel NTV. Another police officer was injured.
The bomb was left in a rubbish bin in a tea garden in the town of Catak, in Van province, said another official.
Both officials spoke on condition of anonymity because of rules that bar civil servants from speaking to journalists.
Television footage showed medics carrying the injured from ambulances into the emergency service of a heavily guarded hospital. One relative could be seen arguing with police as she tried to go in before fainting.
The bomb exploded as police arrived at the scene to investigate a suspicious package at the open-air cafe, where people gather to drink tea, eat snacks and play backgammon.
The blast comes less than a week after a bomb attack at the popular Turkish resort of Antalya killed three people. The Kurdistan Freedom Falcons (TAK) claimed responsibility for that attack.
Police suspect autonomy-seeking Kurdish rebels were behind the bombing, a police official said.
Turkish officials say TAK is a front for the Kurdistan Workers' Party (PKK). The PKK denies this.
TAK has claimed 12 other bomb attacks in urban centres across the country this year in which six people were killed and more than 100 injured.