SAINT PETERSBURG (AFP) - Russian investigators on Thursday (April 6) detained eight people suspected of involvement in the Saint Petersburg metro bombing, as the country held commemorative rallies to honour the 13 victims.

"Six people were detained in Saint Petersburg and two in Moscow who are involved in the act of terror" following an investigation and search operation conducted with the FSB security service and interior ministry, Russia's powerful Investigative Committee said in a statement.

During their searches investigators discovered an explosive device "identical" to the one found at a Saint Petersburg metro station that was discovered shortly before the deadly blast in a metro tunnel.

Firearms and ammunition were also found at the suspects' residences, the committee said.

Investigators said earlier Thursday that they had searched the flat of "several citizens of Central Asian republics, who had been in contact" with suspected bomber Akbarjon Djalilov, thought to be a Russian national born in Central Asian Kyrgyzstan.

The Investigative Committee said objects "relevant to the investigation" had been confiscated but did not disclose their nature.

As the probe into the bombing gained pace, thousands of people gathered at officially sanctioned rallies in Moscow and Saint Petersburg to remember the victims.

People clutching flowers and white balloons gathered at a rally outside Saint Petersburg's Technological Institute metro station, one of the two stations at the heart of the attack.

"I came to show that we will not be frightened," 48-year-old Valentina Zlobina told AFP. "We will become even more united in this tragedy."

In Moscow singers serenaded a crowd of several thousand next to the walls of the Kremlin at an event carried live on state TV that included public transport workers and prominent pro-Kremlin lawmakers.

NO CLAIM

No one has yet claimed responsibility for the blast that tore through a subway carriage in Russia's second city on Monday.

Djalilov's remains were found at the blast site and traces of his DNA were also discovered on a bag containing a bomb at another metro station that was successfully defused, investigators said.

Investigative Committee chief Alexander Bastrykin has ordered officials to look into any potential "links" between the alleged attacker and the Islamic State in Iraq and Syria (ISIS) group.

Militants from ISIS - including foreign fighters from the ex-Soviet Central Asia and the Caucasus region - have repeatedly threatened an attack on Russian soil to avenge Moscow's military backing of Syrian leader Bashar al-Assad.

While the wait continued for any claims of responsibility for the Saint Petersburg blast, ISIS on Thursday said it was behind a separate attack that killed two Russian traffic police in the southern city of Astrakhan this week.

The claim came after security agencies said they had killed four suspects behind the slaying in the city close to the volatile North Caucasus region.

FIRST FUNERAL

At a cemetery outside Saint Petersburg, distraught relatives and friends held the first funeral for one of the victims of the metro attack.

Around 50 people carrying flowers and weeping gathered in a village on the city's outskirts for the funeral of 50-year-old dollmaker Irina Medyantseva who was on the metro with her daughter Yelena, who is now recovering in hospital.

"We had just bought a flat near here in a beautiful building, our daughters were doing well, and then this happened," grieving widower Alexander Kaminsky told AFP.

"You can't process what happened in your mind," said colleague Svetlana Koroleva. "Only a monster could kill innocent people." The attack has shaken the authorities and rattled Saint Petersburg just two months before it hosts the opening game of the Confederations Cup football tournament, a curtain raiser for the 2018 World Cup in the country.

Russia suffered a wave of brutal attacks in the 1990s and 2000s blamed mainly on a rebellion in Chechnya that morphed from a separatist uprising into an Islamist insurgency.

The country's transport network - including the metro in Moscow - was hit repeatedly by suicide bombers, leaving scores dead.

But there had been no attacks against a major city since blasts in the southern city of Volgograd in December 2013, weeks ahead of the Sochi Winter Olympic Games.

ISIS, however, has struck at Russia abroad, claiming a bomb attack in October 2015 that blew up a passenger jet packed with holidaymakers returning to Saint Petersburg out of the air over Egypt, killing all 224 people onboard.