A female suicide bomber on Tuesday killed herself and a Turkish policeman in a strike on the heart of Istanbul's tourist district, the second attack on police to shake city within a week.
The government denounced the bombing as a "heinous terror attack" against the "new Turkey" under President Recep Tayyip Erdogan and said only the bravery of police had prevented a higher toll.
The attack came five days after a member of an outlawed Marxist radical group DHKP-C attacked police on guard outside the Ottoman-era Dolmabahce palace in Istanbul on the Bosphorus.
There was no immediate official indication of a link between the two attacks. But some Turkish media reports said the suicide bomber was a young woman in her mid-twenties who was a member of the DHKP-C.
The woman went into the police station in the Sultanahmet district and told the police in English she had lost her wallet before setting off her explosives, governor Vasip Sahin said on Turkish television.
The Sultanahmet district, which is the home of world famous attractions including the Blue Mosque and Aga Sophia museum, is visited by thousands of Turkish and foreign tourists every day.
One policeman was badly wounded and died of his wounds in hospital, the official Anatolia news agency reported.
A second policeman was lightly wounded. Both were staffing a tourist police post intended to assist tourists with questions and problems.
- 'I lost my wallet' -
Prime Minister Ahmet Davutoglu hailed the bravery of the police for potentially preventing further casualties.
"The bravery of our security forces who sacrificed themselves prevented there being an even larger toll," he said.
Davutoglu said a major investigation was underway to determine which organisation could be behind the attack but refused to speculate further.
"The widest investigation is to be undertaken to determine which organisation could be connected to the attack," he said in televised comments.
Sahin said the woman had posed as a tourist when she approached police in the early evening rush.
"She approached the police at around 5:00 pm (1500 GMT), saying in English: 'I have lost my wallet'. That's all I can say. We are trying to identify the attacker," said Sahin.
"The female bomber lost her life. She has staged the attack by detonating the bomb on her."
Emergency services rushed to the scene and the city tram line that runs through the district was halted.
Davutoglu said two more explosive charges had been found on the corpse of the suicide bomber.
They were blown up in a controlled explosion by bomb experts, shattering nearby windows, according to Turkish news reports.
- 'Heinous attack' -
Deputy Prime Minister Numan Kurtulmus condemned the strike as a "heinous terror attack".
"The target of this attacks is the new Turkey, our beloved nation. But they won't succeed. They won't be able to destroy our brotherhood and unity," he write on Twitter.
The Marxist Revolutionary People's Liberation Party–Front (DHKP-C) said it was behind the January 1 attack outside the Dolmabahce palace, which houses the Istanbul offices of the Turkish prime minister.
The attacker, named as Firat Ozcelik, hurled two grenades at the police honour guard on duty outside the palace but they failed to explode.
The DHKP-C -- a radical Marxist organisation considered a terrorist group by Turkey, the European Union and the United States -- has over recent years carried out a string of sporadic and sometimes deadly attacks in Turkey and abroad.
It claimed a suicide bombing in February 2013 at the US embassy in Ankara where a security guard was killed.
Turkey is also on a high security alert amid the crises in neighbouring Syria and Iraq, where Islamic State (IS) jihadists have seized swathes of territory on the Turkish border.
Western countries have accused Turkey of not doing enough to stem the flow of jihadists through its borders but Ankara insists it has now stepped up frontier security.
Istanbul has been the scene of several bomb attacks in recent years, most notably the November 2003 attacks on two synagogues, the British consulate and a bank that left dozens dead.