Kizilay Square, the scene of the attack on Sunday, is one of Ankara’s most popular commercial and entertainment hubs. The square has been closed fearing a possible second explosion. The Turkish authorities, as they had after other attacks, imposed a ban on local news media coverage of the bombing n an effort to prevent the dissemination of photographs from the bombing scene. On Sunday, Mr. Erdogan said Turkey would continue to assert its right of self-defense against terrorist threats. A statement from the White House said the United States condemned the bombing. This is the third major blast in the Turkish capital since last October. Three weeks earlier, a bombing on a military convoy in Ankara killed 28 people. A militant group based in Turkey called the Kurdistan Freedom Falcons claimed responsibility for that attack, identifying the bomber as a 26-year-old Turkish citizen. The attack on Sunday appeared intended to kill civilians.