According to the NAK, the blast tore through a train at 2.40 p.m. local time (1140GMT) as it was traveling between Sennaya Ploshchad and Technological Institute stations.

The initial reports had put the death toll at 10. An official statement said 9 people had died, but the number of casualties was later raised to 11 in a Health Ministry statement.

An explosive device has been found and deactivated at Vosstaniya Ploshchad metro station, the NAK added.

President Vladimir Putin expressed his condolences to the families of those killed and injured during the explosion, the Kremlin said in a statement.

Putin instructed the city authorities to take "all necessary" measures to help the families of the victims.

"The investigation is ongoing. Of course, we always consider all scenarios, including accidental or criminal action, and above all, those of a terrorist nature," he said. 

Putin said the ongoing investigation will "soon" provide answers regarding the causes of the "tragedy".

Aleksander Kurennoi, spokesman for the Prosecutor General’s Office, said an investigation had been launched into the blast, which he called a "terrorist attack".

Prime Minister Dmitry Medvedev also conveyed his condolences following the explosion. Medvedev said in a statement that Health Minister Veronika Skvortsova was ordered to take "additional" steps to provide injured people with medical assistance.

Meanwhile, Russia announced a three-day mourning after Monday's explosion effective as of Tuesday.

World reactions

Turkish Prime Minister Binali Yildirim extended his condolences to Medvedev over the explosion, and wished a "speedy" recovery to the injured, according to a prime ministry source, who asked not to be named due to restrictions on speaking to the media.

Turkish Foreign Minister Mevlut Cavusoglu tweeted: "I strongly condemn the heinous attack in St.Petersburg that claimed many lives. Condolences to the Russian people." 

Turkish Foreign Ministry said in a statement it was learned with "sadness" that many lives were lost during the explosion. 

"As a country that has expressed its stance against all forms of terrorism on numerous occasions, we strongly condemn this heinous terrorist attack," it added.

The ministry also extended condolences to the "friendly" Russian nation. 

United Nations Security Council on Monday issued a statement condemning "in the strongest terms the barbaric and cowardly terrorist attack".

In a separate statement, Secretary-General Antonio Guterres extended his "deepest" sympathy and condolences to the families of the victims and Russian people.

"Those responsible for this appalling act must be held accountable," Guterres said. 

U.S. President Donald Trump called the attack a "terrible, terrible thing."

"Happening all over the world. Absolutely a terrible thing," he said during brief remarks in the White House's State Dining Room while meeting with Egyptian President Abdel Fattah al-Sisi.

The U.S. Department of Homeland Security said it was "monitoring the situation in St. Petersburg", but said no change had been made to the U.S.'s terror threat level.

German Chancellor Angela Merkel sent a message of condolences to Russian President Putin, according to a press release from her office.

Merkel expressed her sorrow and sympathy for the victims, and said early indications pointed to a "cowardly attack".

"If this proves to be true, that would be a barbarous act which I condemn in the strongest terms," she said.

Merkel also underlined that the masterminds of this suspected terror attack should be found out and held accountable by a thorough investigation.

German President Frank-Walter Steinmeier said in a statement that he followed the news from St. Petersburg with "horror" and "grief".

NATO's  Secretary General Jens Stoltenberg tweeted: "Deepest sympathy to those affected by the #StPetersburg metroexplosion, their loved ones and the Russian people."

"Following the news coming from #SaintPetersburg, together with all EU Foreign ministers. Our thoughts are with all people of #Russia," EU foreign policy chief Federica Mogherini said at her Twitter account.

French President Francois Hollande condemned the attack and expressed solidarity with the Russian people.

"The exact circumstances of this tragedy will be established within the framework of the investigation which has just been opened for the act of terrorism. This tragic event reminds us of the need for constant vigilance," Hollande said in a statement.

U.K. Foreign Secretary Boris Johnson said he was "horrified" by the news of explosion. "My sympathies are with the victims and their families," Johnson tweeted.

U.S. President Donald Trump called the attack a "terrible, terrible thing." 

"Happening all over the world. Absolutely a terrible thing," he said during brief remarks in the White House's State Dining Room while meeting with Egytian President Abdel Fattah al-Sisi.

The U.S. Department of Homeland Security said it was "monitoring the situation in St. Petersburg", but said no change had been made to the U.S.'s terror threat level. 

United Nations Security Council on Monday issued a statement condemning "in the strongest terms the barbaric and cowardly terrorist attack".

UN Secretary-General Antonio Guterres condemned the bombing in the metro and extended his "deepest" sympathy and condolences to the families of the victims and Russian people.

"Those responsible for this appalling act must be held accountable," Guterres said in a statement. 

German President Frank-Walter Steinmeier said in a statement that he followed the news from St. Petersburg with "horror" and "grief".

German Chancellor Angela Merkel sent a message of condolences to Russian President Putin, according to a press release from her office.

Merkel expressed her sorrow and sympathy for the victims, and said early indications pointed to a "cowardly attack".

"If this proves to be true, that would be a barbarous act which I condemn in the strongest terms," she said.

Merkel also underlined that the masterminds of this suspected terror attack should be found out and held accountable by a thorough investigation. 

French President Francois Hollande condemned the attack and expressed solidarity with the Russian people.

"The exact circumstances of this tragedy will be established within the framework of the investigation which has just been opened for the act of terrorism. This tragic event reminds us of the need for constant vigilance," Hollande said in a statement. 

Secretary General of the Council of Europe Thorbjorn Jagland said he was "shocked and saddened by the news of deadly blasts in St. Petersburg."

"I extend my sincere condolences to our member state Russia," Jagland tweeted.

Pedro Agramunt, President of the Parliamentary Assembly of the Council Europe (PACE), strongly condemned the attack. 

"We are confronted once again with the barbarism and indiscriminate violence which are the marks of terrorism. This unspeakable act, targeting innocent people, is totally contrary to the values and principles of the Council of Europe," Agramunt said.  

"More than ever before, we must pool our efforts to combat terrorism and violent extremism effectively and provide a bulwark against the hatred which fuels them," he added. 

Greek Foreign Ministry said in a statement that it was "shocked" by the "deadly" blast in St. Petersburg. 

"Greece stands by the Russian people and their government, sends sincere condolences to the families of the victims and wishes a speedy recovery to those wounded," it added.

Azerbaijani President Ilham Aliyev said in a statement that he was "deeply" shocked with the incident. 

"We strongly condemn any action directed against innocent civilians," said Aliyev. 

Kazakhstan’s President Nursultan Nazarbayev conveyed condolences to Putin in a telegram, the presidency said in a statement. 

"We are shocked by this tragic news and we sincerely share the pain of loss together with the people of the Russian Federation,” Nazarbayev said.

Kyrgyz President Almazbek Atambayev condemned in a statement the "inhuman" act aimed at "peaceful" and "innocent" citizens.

Uzbekistan's President Shavkat Mirziyoyev extended condolences to Putin in a statement.

Anadolu Agency