Ian McKellen said it was a point of deep "principle" as he joined hundreds of protesters outside the Russian embassy in London voicing their anger against a "purge" of gay men in Chechnya.
The actor was among scores of people outside the embassy near Kensington Gardens on Friday to demand that Russia takes decisive action against those responsible for a brutal persecution of gay men earlier this year.
In April reports emerged of gay people being abducted, tortured and even killed in an anti-gay "purge" in the southern Russian federal republic.
Russian independent newspaper Novaya Gazeta reported that about 100 gay men were rounded up, tortured and killed by police, with others held in detention centres.
British Foreign minister Alan Duncan raised the issue in the UK's House of Commons in condemnation of the attacks, calling it "beyond contempt" and promising to raise concerns with allies in Europe and the Commonwealth.
Despite international condemnation, Chechen and Russian authorities initially denied the accusations, Amnesty International said, with a Chechen spokesman denying that LGBTI people even exist in the region.
But defiant protesters made their anger known outside the Russian embassy in Kensington, London, to tell ambassador Alexander Yakovenko "we exist".
Wearing black T-shirts, hundreds of people waved giant rainbow LGBTI rights banners and carried placards with the simple message "love is a human right".
Addressing the protest, McKellen - one of the founders of LGBTI rights group Stonewall, which organised the demonstration with Amnesty - told fellow campaigners: "I'm not usually in favour of commenting or interfering on the internal affairs of another country, even when it's as close to us as Chechnya is, but the point is that this is a matter of principle.
"Our principles are shared across borders, and the plight of the gay men in Chechnya is the plight of gay men and women throughout Russia."
Reading a message from the Russian LGBT network to the crowd, he said: "Right now, we need you to demand justice, we need you to tell your governments to take action, we need you to accept refugees, we need you to call for a transparent and just investigation that is going to hold those responsible to account."
The protest was one of 27 held at Russian embassies around the world to put pressure on authorities to act and protect LGBTI communities.
A petition bearing 177,000 names was taken to the London embassy - but Amnesty said staff at the building refused to take it, despite being told about it in advance.
After the protest, McKellen kissed fellow Stonewall founder Michael Cashman on the cheek as they laid rainbow roses on a rainbow flag - the symbol of the LGBT movement - outside the embassy.
Speaking to the Press Association, McKellen, who has just returned from a visit to Russia to meet LGBT people there, said Russia had a "moral responsibility" to intervene in Chechnya.
He said: "It has become absolutely clear now, there is no doubt these atrocities are happening.
"This is more than an internal affair, this is a principle - that gay people should be treated equally with the rest of society.
"That's what we fought for here, that's what we've achieved. It's what doesn't apply in Russia any longer, I'm afraid, and certainly not in Chechnya.
"So, to our gay friends and our straight friends in the federation of Russia, we are sending love and solidarity and concern, and we'll be putting pressure on our own Government to make proper representations from a country like ours where gay people are free to speak their mind and live their lives."
Condemning the stance of Chechen authorities on gay people, he said: "It is possible to think that gay people don't exist because gay people are so frightened that they daren't say they exist.
"What a condemnation it is of Chechnya that its authorities should believe that there are no gay people there, and if there were they shouldn't be - it's absolutely appalling.
"If gay people are invisible it's because they are frightened to be themselves and come out, so it's a condemnation not of gay people but the society they are trying to exist in."
Citing his love of the playwright Anton Chekhov and the homosexual composer Tchaikovsky, McKellen added: "We're perfectly ordinary people going about our lives. We love Russia ... as a friend, as a neighbour and a supporter of all things Russian I'm here to say 'Please, do the right thing, and understand how cruel you're being'.
"The authorities in Russia do have power over the Chechen state and really do have a moral responsibility - that's what's on trial here."
Amnesty International said it was told by the Russian embassy earlier this week it was not allowed to submit its petition.
The charity said it tried again on Friday, asking police outside to check with officials if the embassy would accept the petition through them, but was again told no.
But a spokesman for the Russian embassy denied ever being approached.
He said: "Nobody tried to hand in any petition to any member of staff today, therefore the embassy did not refuse to take any petition.
"Anybody who wants to submit one can do so over the post at any moment."
Amnesty International said it would have the petition delivered by courier.