MOSCOW, Mar. 25 (Reuters) - Russia lowered flags to half-mast for a day of mourning and charged four men it accused of gunning down scores of people at a concert outside Moscow on Friday night in the deadliest attack inside Russia for two decades.

President Vladimir Putin declared Sunday a national day of mourning after pledging to punish all those behind the attack, in which 137 people were killed, including three children, and 182 were injured.

Over 100 people remained in hospital, some of them in a serious condition. Video footage showed a sombre-looking Putin lighting a candle at a church at his residence outside Moscow on Sunday evening to honour those who died.

Earlier on Sunday, people laid flowers at Crocus City Hall, the 6,200-seat concert hall outside Moscow where four armed men burst in just before Soviet-era rock group Picnic was to perform its hit "Afraid of Nothing". The men fired their automatic weapons in short bursts at civilians who fell screaming.

Islamic State has claimed responsibility for the attack, but Putin has not publicly mentioned the Islamist militant group in connection with the attackers, who he said had been trying to escape to Ukraine. He said that some on "the Ukrainian side" had been prepared to spirit the gunmen across the border.

Ukraine has denied any role in the attack.

Moscow's Basmanny district court on Sunday charged four suspects with acts of terrorism in connection with the attack, naming them as Dalerdzhon Mirzoyev, Saidakrami Rachabalizoda, Shamsidin Fariduni, and Muhammadsobir Fayzov, according to Moscow courts' official Telegram channel.

It said the men, identified by Russian media as all being citizens of the ex-Soviet republic of Tajikistan living in Russia, would be remanded in pre-trial custody until May 22. Three of the four had pleaded guilty to all charges, it said.

After unverified and brutal videos of the suspects' interrogations circulated on social media, courtroom images published by Russian media showed one suspect was brought in on a wheelchair apparently missing an eye, another had a bandage where his right ear should be, another had a black eye and a ripped plastic bag around his neck, and a fourth suspect with a swollen face seemed disoriented and struggling to keep his eyes open.

Friday's attack was the deadliest on Russian territory since the 2004 Beslan school siege, when Islamist militants took more than 1,000 people hostage. More than 300 people died at the time, more than half of them children.

Dmitry Medvedev, deputy chairman of Russia's Security Council, said Russia would target those behind the deadly shooting wherever they were from and whoever they were.

He had previously spoken of the need to meet "death with death" and some lawmakers have begun to discuss whether the death penalty should be re-introduced.

Across Moscow, billboards carried a picture of a single candle, the date of the attack and the words "We mourn".

Countries around the world have expressed horror at the attack and sent their condolences to the Russian people.

Photo from Reuters