Plaid Cymru president Dafydd Iwan and Welsh assembly presiding officer Lord Dafydd Elis Thomas are to pay tribute to Gwynfor Evans at his funeral.
Gwynfor Evans was Plaid Cymru president for 36 years
The service for the former Plaid Cymru president at Aberystwyth's Seion Chapel on Wednesday will also be open to members of the public.
Mr Evans, Plaid Cymru's first MP and its leader for nearly four decades, died at his home on Thursday, aged 92.
His family will hold a private service at home before the funeral at 1330 BST.
In a statement giving details of the funeral they said: "We would like the service in Aberystwyth to be an opportunity for the people of Wales give thanks for Gwynfor's life."
A large crowd of mourners is expected, and the service is to be broadcast through a sound system the for those who are unable to get into the chapel.
Mr Evans' minister, Reverend Huw Roberts, will preside at the service, with the Reverends Andrew Lenny and Vivian Jones also taking part.
Former Plaid Cymru MP Dafydd Wigley will join the political figures paying tribute, along with several of Wales' political and Christian leaders.
Following Mr Evans' wishes, his family have asked for any donations to go to Christian Aid "as a contribution from the people of Wales to the people of the world".
A private family service will be held at Aberystwyth Crematorium later in the day.