The Investigative Committee said it was holding two suspects -- a maintenance foreman, Valery Bashkatov, and his assistant Yury Gordov -- and added that high-ranking officials could also be arrested.

"The detained have already been questioned," the committee said, noting the men will soon be formally charged with safety breaches.

The train derailed between Park Pobedy and Slavyansky Boulevard on the blue line.

The Investigative Committee said that the metro had since May been carrying out work to install a set of points -- a section of track allowing trains to change lines -- to launch a section of new tracks.

The suspects had overseen the works, said the committee, which reports directly to President Vladimir Putin.

The committee said in a statement it believed "the works have not been conducted in a proper manner."

"A set of points was fixed in place with a piece of regular 3-millimetre wire which snapped."

The Investigative Committee indicated that higher-ranking officials behind the crash could also be arrested, saying it intended to probe "absolutely everyone involved in this tragedy."

Putin - currently in Brazil for a BRICS summit - on Tuesday had ordered a criminal probe in the crash.

Moscow mourns

City authorities declared Wednesday a day of mourning as calls mounted to urgently improve the marble-clad but overcrowded metro, one of the world's busiest.

A hospital spokesperson reported today that another victim had died from their injuries, bringing the death toll to 23.

“One patient who was in an extremely severe condition has died today in intensive care,” the representative said at a teleconference in the emergencies ministry.

Flags in Moscow are flying at half-mast, and entertainment events have been cancelled. Special prayer services have been taking place at all the city churches.

Sixteen hospitals have opened special blood donor centers. Many crash victims are in critical condition at hospitals, with traumatic brain injuries and bone fractures, chest and abdominal injuries.

The city authorities is to pay $90,000 compensation to the families of those killed.

The Moscow mayor has given instructions to conduct unscheduled checks of all the metro facilities.

The subway, which first opened in 1935 under Stalin, transports some 9 million passengers every day.

Flowers by a sign at the metro entrance which says  'Sorry, station closed for technical reasons' (RIA Novosti)