They came in the hope of enjoying Istanbul's sights in the relative peace of the low season. But Tuesday's suicide attack in the heart of the city's historic Sultanahmet area left tourists reeling.

'This is the first time that I've been afraid for my life while on holiday," said French tourist Nathalie Julien after the attack blamed by Turkish authorities on ISIS group.

"I can't believe this is happening here, in the heart of Istanbul. It's like you get invited to someone's house, treated really well and, all of a sudden, get attacked," Julien told AFP.

"I just want to get home now, it's scary," she said, dragging her luggage into a tram.

Ten people, mostly Germans, were killed early Tuesday when a Syrian suicide bomber blew himself up in Sultanahmet Square, next to the world famous Ottoman Blue Mosque and Byzantine-era Hagia Sophia museum.

The target appeared to have been chosen carefully, with the square acting as a gathering point for tens of thousands of tourists every day.

The foreigners killed appeared to be taking advantage of the morning calm to take in the sights before the afternoon rush.

Germans are by far the most frequent foreign vistors to Turkey, with 258,613 arriving in November 2015, accounting for 15 percent of all arrivals.

Some 5.4 million Germans arrived in Turkey between January and November last year.

Many of the tourists still flocking to museums and kebab restaurants in the area a few hours after the attack were still in a state of shock.

Czech couple Zdenek and Eva, both 60, said they were planning to spend a lazy afternoon in the neighbourhood when they heard the huge blast that sent people screaming and running for cover.

"Because it was so sunny, we wanted to stay outdoors, so we skipped the mosques and museums and then this horrible thing happened," Zdenek said, sitting at a cafe, sipping his tea.

"I came to Istanbul many times before and always thought it was a very safe place, but it seems nowhere is safe anymore," he said.

"I think they chose such a day intentionally to hurt as many people as possible. It's truly horrible," Eva said.

Adrienn Martin, a 28-year-old student from Hungary, considered herself lucky to have opted to visit the Grand Bazaar with her boyfriend instead of hanging around Sultanahmet Square, where her hotel is located.

She said it was her second near miss after escaping the 2004 Indian Ocean tsunami while in India.

"It's quite shocking. Two days ago we were sitting at the same spot where the explosion happened," she said. "It really got my nerves, seeing ambulances around, when I was expecting a memorable day.

"I'm terrified. It could have been us," she said, clutching her boyfriend's hand.

Turkey had been on high alert after a series of attacks blamed on ISIS, including a double suicide bombing in October in Ankara that killed 103 people.

But this is the first time in recent years that tourists have been directly targeted. Previous attacks blamed on ISIS in Turkey had all hit pro-Kurdish targets.

"I had just stepped out of my hotel when I heard the explosion. I thought our hotel had been bombed," said Stefan, from Austria.

"Watching all the news, I was already hesitant about visiting Istanbul because of the terror risk. My fears have now been justified," he said.