LONDON, May 6 (Reuters) - President Vladimir Putin will send a "doomsday" warning to the West when he leads celebrations on Monday marking the 77th anniversary of the Soviet Union's victory over Nazi Germany, brandishing Russia's vast firepower while its forces fight on in Ukraine.

Defiant in the face of deep Western isolation since he ordered the invasion of Russia's neighbour, Putin will speak on Red Square before a parade of troops, tanks, rockets and intercontinental ballistic missiles.

A fly-past over St Basil's Cathedral will include supersonic fighters, Tu-160 strategic bombers and, for the first time since 2010, the Il-80 "doomsday" command plane, which would carry Russia's top brass in the event of a nuclear war, the Defence Ministry said.

In that scenario, the Il-80 is designed to become the roaming command centre for the Russian president. It is packed with technology but specific details are Russian state secrets.

Ahead of May 9, speculation swirled across Moscow and Western capitals that Putin was preparing some sort of special announcement on Ukraine, perhaps an outright declaration of war or even a national mobilisation.

Kremlin spokesman Dmitry Peskov dismissed those suggestions on Wednesday, describing them as "nonsense".

The Kremlin did not respond to requests for comment on what Putin might say in his speech, to be delivered from the Red Square tribune in front of Vladimir Lenin's Mausoleum.

Last year, Putin jabbed at Western exceptionalism and what he said was the rise of neo-Nazism and Russophobia - trends he has returned to again and again when addressing the issue of Ukraine.