Seven people were killed in a bomb attack on the government building in the capital, Oslo, before a gunman opened fire at a youth camp on the tiny island of Utoeya, killing at least 84 people.
A 32-year-old man believed to be a Christian fundamentalist has been arrested by police in connection with both attacks.
The man is being named by Norwegian press as Anders Behring Breivik and is believed to have links with right-wing extremists.
Norwegian Prime Minister Jens Stoltenberg was not injured in the Oslo attack but had been due to visit the camp a few hours after the shooting.
Media reports say that the gunman was dressed as a police officer and even shot at camp delegates as they tried to swim away from the island.
Dr Tveit, a Norwegian Lutheran pastor, had just left Oslo when he learned of yesterday's bombing.
He expressed his sorrow over both attacks and called on the church worldwide to pray for Norway.
"Attacking the core institutions of a democratic society and innocent youth gathered for a workshop to discuss political issues, leaves me shocked," he said.
"Being close to these events, I am deeply saddened, realising that this has happened in my beloved country, with its people, its leaders, and its institutions.
"In times like this the Norwegian people and government need to the solidarity of the international society and the prayers of the worldwide church."