The Archbishop of Canterbury said the Church of England still has a "long way to go" as he spoke of the scars and "hurt" suffered by campaigners in favour of women's ordination in the face of "knee-jerk" resistance to change.
The Most Rev Justin Welby urged celebration with "boundless fullness of heart and no holding back" - declaring it is "birthday time, party time" on the 20th anniversary of the ordination of the first women to the priesthood in the Church of England.
But he told more than 2,000 people, including 700 women ordained as priests in 1994, their family and supporters gathered at a national service in St Paul's Cathedral, that they should not overlook the cost to those who had made the change possible.
He said there was "much to celebrate" on the anniversary but the "sacrificial commitment" of those who achieved change should be remembered.
"In our celebrations let us not overlook the cost, the bitterness of disappointment and rejection, the knee-jerk resistance of an institution facing change," he said.
He added: "As a representative of that institution, I want to thank those here today whose costly loyalty, whose scars, make this celebration possible, and to say personally how I grieve that it cost so much, to apologise for my own part in that hurt."
The Archbishop said the service was one of celebration but "never of complacency".
"As we celebrate how far we have come, let us be mindful of the distance yet to travel," he said.
"In 20 years we have come a long way - how did we not see that women and men are equally icons, witnesses, vessels of Christ for the world?"
The service was presided over by the Rev Canon Philippa Boardman, canon treasurer of St Paul's, with Archbishop Welby choosing to act as a deacon.
Women who were ordained as priests in 1994 processed into the cathedral at the start to prolonged applause and a standing ovation.
The service was relayed live on a screen into nearby Paternoster Square, with Holy Communion brought into the square for outdoor worshippers.
The service heard testimonies from the Very Rev June Osborne, Dean of Salisbury, and the Rev Kate Boardman, assistant curate at St Mary's Heworth in Gateshead, who was ordained by Archbishop Welby when he was Bishop of Durham.
The retired Bishop of Bristol, the Rev Barry Rogerson, who ordained the first set of women priests in 1994, gave a reading at the service.
The Church of England voted in favour of women's ordination in 1992.
The decision to ordain women sparked a walkout by traditionalists opposed to female ordination.
Today's event came as the General Synod prepares to give final approval to legislation introducing the first women bishops in July. The move could mean the first woman bishop is appointed by early next year.
The Very Rev Osborne told the service that women priests in 1994 knew they had crossed a "new threshold".
"The nation cheered on the Church of England. The vocation of being the first is superficially exciting - how many times have we heard it said to us 'So you are one of the first'?
"Yet being the first also holds its own demands. We embodied change."
The Rev Boardman said as an undergraduate she had seen a notice in a church warning "This parish will take no part in the apostasy of women".
She joked to the service: "Truthfully, I had to look up apostasy."
"Truthfully, I would have laughed at the idea of me becoming a priest," she added.
She said she had been ordained by Archbishop Welby to "cause trouble with his blessing".
"Today I would laugh at the idea of being anything but a priest," she said.
The Archbishop later quipped: "I don't actually remember ordaining you to cause trouble - but I don't have much of a memory."
Roman Catholic campaigner in favour of women's ordination Lala Winkley, 70, a retired drama teacher from London, held a placard inside the cathedral at the start of the service.
She was one of a group from the Catholic Women's Ordination (CWO) campaign attending the service.
She said she felt "deep, deep sadness" at the Roman Catholic Church's refusal to ordain women priests.
"I am a Catholic and I am not envious of anything but I grieve for our particular tradition because it is so bereft of women's input and significantly the poorer because of it - and it does not realise it," she said.