At least 95 people have been killed in twin explosions in Turkey's capital Ankara, targeting activists gathering for a peace rally organised by leftist and pro-Kurdish opposition groups.

Turkish prime minister Ahmet Davutoglu said the blasts were likely carried out by two suicide bombers, and declared three days of national mourning.

It is understood 246 people are still being treated for injuries with 48 of them in intensive care, the prime minister's office said.

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Turkish president Tayyip Erdogan condemned the attacks and called for solidarity.

"I strongly condemn this heinous attack on our unity and our country's peace," Mr Erdogan said in a statement posted on the presidency's website.

Bodies of those who died at the scene were covered by flags and banners, including those of the pro-Kurdish opposition Peoples' Democratic Party (HDP), with bloodstains and body parts scattered on the road.

The area was to have hosted a peace rally organised by leftist groups later in the day, including the HDP, calling for an end to the fighting between Kurdish separatists and the Turkish government.

"Many of the injured people are heavily injured, so there is a fear that the number of dead people may increase," the party said in a statement.

Witnesses said the two explosions happened seconds apart shortly after 10:00am (local time) as hundreds gathered for a planned "peace" march to protest against the conflict between Turkish security forces and Kurdish militants in the south-east.

"I heard one big explosion first and tried to cover myself as the windows broke. Right away there was the second one," said Serdar, 37, who was working at a newspaper stand in the train station.

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"There was shouting and crying and I stayed under the newspapers for a while. I could smell burnt flesh."

There were scenes of chaos after the blasts, as ambulances raced to get to the wounded and police cordoned off the area around the train station.

Bodies lay in two circles around 20 metres apart where the explosions had taken place.

Those involved in the peace march tended to the wounded lying on the ground, as hundreds of stunned people wandered around the streets.

"We heard one huge blast and then one smaller explosion and then there was a great movement and panic. Then we saw corpses around the station," Ahmet Onen, 52, said.

"A demonstration that was to promote peace has turned into a massacre, I don't understand this."

There have been no claims of responsibility for the attack.

Turkish police fired in the air to disperse demonstrators angered by the deaths of their fellow activists from the scene.

The United States condemned the bombing, calling it a heinous terrorist attack on peaceful demonstrators.

"In light of the ongoing violence in Turkey and the region, it is particularly important at this time that all Turkish citizens recommit to peace and stand together against terror," the US State Department said in a statement.

Many Twitter users in Turkey were unable to access the social media network on Saturday.

It was not immediately clear if the problem was related to the attack, but the Turkish government has moved before to block or limit social media sites, including Twitter, at sensitive times, prompting accusations of censorship.

'We are faced with a murderous state'

Thousands of people later marched down Istanbul's main central avenue to protest the attack, with police maintaining a heavy presence but not intervening, an AFP photographer said.

Some carried placards reading "the state is a killer" and "we know the murderers", echoing allegations of state responsibility made in the aftermath of the attack but denied by the government.

Thousands of protesters at the Istiklal avenue in Istanbul later took part in a march against the deadly attack. ( AFP: Ozan Kose )

Protests were also taking place in several other Turkish cities including Izmir on the Aegean and south-eastern cities such as Batman and Diyarbakir, the Dogan news agency reported.

Mr Erdogan cancelled his engagements to consult with senior security and government officials, while HDP leader Selahattin Demirtas and the leader of the main opposition CHP, Kemal Kilicdaroglu, also cancelled their plans for Saturday.

"Like other terror attacks, the one at the Ankara train station targets our unity, togetherness, brotherhood and future," he said in a statement, calling for "solidarity and determination".

Mr Demirtas said he blamed a "mafia state" in the country for the twin explosion.

"We are faced with a murderous state which has turned into a mafia and a state mentality which acts like a serial killer," he said.

"Is it possible that a state with such a strong intelligence network did not have prior information on the attack?"

He drew a parallel with the bombing of an HDP rally in the south-eastern city of Diyarbakir on the eve of the last election in June and a suicide bombing blamed on Islamic State in the town of Suruc near the Syrian border in July, which killed 33 mostly young pro-Kurdish activists.

Prime Minister Malcolm Turnbull expressed sympathies following the "despicable act of cowardice" on behalf of Australia.

"I was shocked and deeply saddened to hear of the attack in Ankara yesterday which resulted in the deaths of so many innocent civilians exercising their democratic rights," he said in a statement.

"On behalf of the Government and the people of Australia I extend my sincere condolences to Turkey on this tragic loss of life.

"All Australians are shocked by this despicable act of cowardice. Our thoughts and prayers are with the families and friends of the victims who lost their lives."

The attack comes with Turkey on edge ahead of November 1 polls and a wave of unrest over recent months.

Fighting has resumed between the militant Kurdistan Workers' Party (PKK) and the Islamist-based government forces after the collapse of a two-year ceasefire in July.

Over 140 members of the security forces have since been killed while Ankara claims to have killed over 1,700 Kurdish militants in a relentless bombing campaign.

People attending the peace march have tended to the wounded on the streets. ( AFP: Adem Altan )

Reuters/AFP