Liam Neeson, Stephen Fry and Graham Norton have called for the introduction of same-sex marriage in Northern Ireland ahead of a planned demonstration there.
Snow Patrol singer Gary Lightbody and Fall star Bronagh Waugh have also added their voices to the campaign.
Saturday's parade in Belfast city centre comes amid an ongoing political dispute on the North's ban on same-sex marriages.
The issue is one of the logjams in the ongoing negotiations to restore powersharing at Stormont, while the DUP's opposition to a law change has attracted increased scrutiny across the UK since the party became the British Government's kingmaker at Westminster.
Amnesty International, which is one of the leading members of the Love Equality coalition organising the march, released statements from the celebrity backers.
"We've had enough of a history in our society in Northern Ireland of discrimination, mistrust and hatred," said Neeson.
"Yet, we're also an open-hearted, welcoming and terrific people. Let us show that to the world by treating gay, lesbian and transgender peoples as our brothers and sisters and allowing them to marry, if they so wish. By joining the rest of Ireland and other western countries in celebrating equality we can also attract more investment to our province.
"We have shown the world how we can affect a peace accord when many thought it impossible (1998 Good Friday Agreement).
"We must act on this and welcome all members of society through civil marriages, while still keeping our Christian values. Love, is love, is love."
Northern Ireland is the only part of the UK and Ireland where same-sex marriage remains outlawed.
The DUP has used a controversial Stormont voting mechanism - the petition of concern - to prevent a law change, despite a majority of MLAs supporting the move at the last vote.
The party rejects any suggestion it is homophobic, insisting it is instead protecting the "traditional" definition of marriage.
Following March's snap Assembly election, the DUP no longer has the electoral strength to deploy a petition of concern in its own right, though it could still potentially combine with other socially conservative MLAs to do so.
That will only be tested once, and if a devolved Assembly can be re-established out of the current political crisis in Belfast.
Norton said the historic referendum that approved same-sex marriage in the Republic of Ireland was a "proud gay day".
"It moved me to tears because it was something I never dared hope would happen," he said.
"My own mother was from Northern Ireland, so of course I have a huge affection for the place and its people.
"I know it is hugely frustrating for gay people there that it is the last part of these islands still without marriage equality. Especially when there is such overwhelming support for it among the public.
"My heart is with all those who will be marching for civil marriage equality in Belfast on July 1st. Go you."
Fry, who married comedian Elliott Spencer in January 2015, said: "I send my love to the people of Northern Ireland. Especially all those couples striving and marching for the right to marry and have their love celebrated.
"Why should loving couples in Northern Ireland be denied this chance of happiness, of equality, enjoyed by people in every other part of these wonderful islands? It doesn't make sense.
"Simply put, I want the same joy that I have found to be available to people in Northern Ireland."
Co Down-born singer Lightbody added: "Two years ago, I marched with 20,000 fellow Northern Ireland citizens for marriage equality. It was a beautiful day of hope, joy and solidarity and I was so proud to be from 'Norn Iron'.
"Two years on and somehow - defying reason given it's the will of the people - we still do not have marriage equality in Northern Ireland.
"We're in the States making our new album, so we can't be there in person to march again for marriage equality, for togetherness, for reason and for love. Everyone in Snow Patrol will be there in spirit.
"I hope the day is as special, even more special, than two years ago and I hope those that should listen to the will of the people do listen. Surely, they must. Surely. One love."
Coleraine-born actress Waugh will host the parade rally at Belfast City Hall.
"With the spotlight being shone on Northern Ireland at the minute, now is our chance to really step up our campaigning a gear and give Northern Ireland what it so clearly wants and deserves, finally - equal marriage," she said.
"This is a human rights issue. We will not rest until everyone in Northern Ireland has equal rights like the rest of the UK."
Singer Neil Hannon of The Divine Comedy also backed the campaign.
"I am 100% behind the fight for marriage equality in Northern Ireland," he said.
"The inexorable march towards equality and acceptance of all sexual orientations and genders in recent decades has been a joy to behold.
"Many brave people have put themselves in the firing line in pursuit of the dream.
"Thankfully they are now claiming victory after victory across Europe and the wider world. I fervently hope for the same in our little part of that world.
"In the immortal words of Lenny Kravitz - Let Love Rule. Have a great day."