From one of the priests of our church:
If you believe in the power of prayer or you used to believe in the power of prayer or you have some sort of faith--any kind of faith--that God or some Higher Power or some Positive Energy can help us, this is the moment to say such prayers. If you do not know what to say, then just repeat the words, "May God help us to become the nation we were meant to be: a nation of hope and justice and peace. Help us to emerge from the darkness into the light."
If you are a religious person of any sort, then here is a prayer from The Book of Common Prayer that you might want to say:
"Grant, O God, that your holy and life-giving Spirit may so move every human heart and especially the people of this land, that by faithful administration and wise laws, the barriers which divide us may crumble, suspicions disappear, and hatreds cease; that our divisions may be healed, and that we may live in justice and peace. Amen."
Amen.