Traditionalists in the Roman Catholic church have expressed concern after Pope Francis became the first pontiff to wash the feet of two women during a Maundy Thursday mass, a move liberals welcomed but some conservatives feared set a worrying precedent.

At the Casal del Marmo youth detention centre on the outskirts of Rome, the Argentinian pope washed and dried the feet of 12 inmates as part of the traditional rite representing Jesus's final act of humility towards his disciples.

He had surprised the Vatican with his decision to wash the prisoners' feet – a move that echoed the early years of John Paul II, who once performed the rite in the St John Lateran basilica with a dozen homeless men.

But it was his inclusion of two young women, as well as Muslims, in the ceremony that was the most dramatic break with tradition. It even caused some traditionalists to wonder openly whether Francis, who is doctrinally a theological conservative who has explicitly stated he is against female ordination, might one day be willing to open the priesthood to women.

The Vatican's spokesman Federico Lombardi insisted the rite took place in "a specific situation in which excluding the girls would have been inopportune in light of the simple aim of communicating a message of love to all".

Chris Gillibrand, a British commentator, wrote on his blog, CathCon: "We will see whether it is a particular case, as Lombardi suggests, or the beginning of a journey. Given his active support for the charismatic movement in his diocese, one can only be concerned that he could be prepared to ordain women … How can the pope maintain discipline in the church if he himself does not conform himself to prevailing ecclesiastical legislation?"

This was not the first time Francis had washed female feet. As Cardinal Jorge Bergoglio and archbishop of Buenos Aires, he often included women in the gesture. However as pope, his move was quietly groundbreaking. In their later years John Paul II and Benedict XVI had restricted the rite to 12 Catholic priests.