The blast outside one of the oldest shopping malls in Ankara hurled glass and other debris over a wide area. A body, covered in a white sheet, lay outside the building.
Prime Minister Recep Tayyip Erdogan said four Turks and one Pakistani were killed in the blast, and that authorities were investigating the type of bomb used.
"Is it a suicide bomber or a parcel bomb? Technical teams are working on this," Erdogan told reporters after inspecting the scene.
Pakistani embassy spokesman Abdul Majeed Niazi said eight Pakistanis were injured, but that there was no Pakistani fatality. It was unclear why his account conflicted with that of the prime minister.
Health Minister Recep Akdag said around 80 people were injured, most of them slightly.
Private NTV television, quoting police sources, said the bomb was made of plastic explosives, the type favored by separatist Kurdish rebels. Militant leftists and Islamic extremists also operate in Turkey.
There was no immediate claim of responsibility. Erdogan said speculation about the identity of the perpetrators would only serve the cause of the terrorists.
Television video shortly after the blast showed medics tending to the injured, bloodied and with some of their clothes torn away, and carrying them to ambulances on stretchers.
"We were cleaning the windows when the pressure from the blast pushed us to the ground," said Murat Coskun, the owner of a shop that sells cellular telephones near the explosion site. "Everything was covered in dust. I could only hear people screaming."
"The scene here is horrific," Mayor Melih Gokcek said.
Forensic teams collected evidence.
"I rushed to the site after the explosion, several people were lying on the ground," said Mahmut Dursun. "It was like a war zone."
The explosion occurred as the city hosted an international defense industry fair, with some 400 companies from dozens of countries displaying their goods.
There is growing impatience in Turkey over how to deal with Kurdish rebels, and the government has not ruled out military operations aimed at their bases in northern Iraq. The United States opposes Turkish military action in Iraq, fearing it would complicate U.S. efforts to restore stability there.
In recent weeks, Turkey has endured political turmoil pitting the Islamic-leaning government against the military-backed secular opposition, which staged huge anti-government rallies. The ruling party's presidential candidate was forced to abandon his bid for the office, and early general elections were called for July 22.
The neighborhood where the blast occurred is Ulus, the oldest part of the city. It lies near museums and the old parliament house. The shopping mall that was damaged is called Anafartalar and sells mostly clothing and textiles.
In 2003, al Qaeda suicide truck bombers attacked two synagogues, the British Consulate and a British bank in Istanbul, killing 58 people.
In September, suspected Kurdish rebels set off a bomb at a bus stop in Turkey's largest majority Kurdish city of Diyarbakir, killing 10 people. A month earlier, a hard-line Kurdish militant group claimed responsibility for a bus bombing in the Mediterranean resort town of Marmaris that injured 20 people, including 10 Britons.