LONDON (AP) _ Mstislav Rostropovich, the Soviet-born cellist, and flutist James Galway will join other leading musicians in a charity concert tonight for victims of the Armenian earthquake.

The concert will be televised in six countries, including the Soviet Union, and broadcast live to an estimated 25 million listeners by the British Broadcasting Corp.'s World Service, according to a sponsor, The Times newspaper, and the BBC.

Prince Charles, heir to the British throne, and his wife Princess Diana were expected to be among the 2,000 specators at the sold-out Barbican Hall.

Organized by the British Association of Concert Agents, the concert is part of a ''Musicians for Armenia'' appeal, which expects to raise $452,000 from the concert, from donors, including 450 musicians worldwide, and from the film and recording rights to the concert.

Conductor Andre Previn, who will lead the English Chamber Orchestra at the concert, said at a rehearsal this afternoon that it was ''amazing'' that the musicians had been free. ''In our business everyone's booked up for the next two or three yers,'' he said. ''It was just good fortune that everyone could make it at such short notice.''

Rostropovich, who gave his first solo concert in Britain in nearly two decades on Friday also in aid of Armenia, will play Villa-Lobos' Bachianas Brasileiras No. 1. He will also play the piano in accompanying his Soviet-born wife, soprano Galina Vishnevskaya, in songs by Tchaikovsky. Rostropovich is of Armenian descent but an American citizen.

The program will include Galway playing Dubussy's Syrinx and Previn conducting the English Chamber Orchestra in Beethovan's Coriolan Overture.

Soviet pianist Andrei Gavrilov, whose family in Armenia escaped injury in last week's quake, will play Mozart and Chopin.

The musicians will also include violist Yuri Bashmet, pianist Barry Douglas and violinist Cho-Liang Lin.

Everyone connected with the event, from program printers to Barbican staff and the musicians, donated their services.

The income plus the entire cost of the tickets, except tax, was to go to the British Red Cross Armenian Earthquake Appeal fund.

Tickets ranged in price from $18 to $45.

The Times reported that the concert was to be televised in Spain, the Netherlands, Ireland, Australia, Belgium and the Soviet Union.