A memorial service at Oslo Cathedral was joined by members of the royal family and Prime Minister Jens Stoltenberg.
Mr Stoltenberg told victims' families: "We're crying with you."
He said: "Every single one of those we have lost is a tragedy in itself. Together the loss is a national tragedy.
"We are still struggling to understand the extent of this tragedy.
"Many of us knew somebody who is now gone.
"Even more know of someone.
"I knew several of them."
He left floral tributes along with crowds of ordinary citizens who gathered outside the Domkirke cathedral to pay their respects.
A service was also held in the Norderhov Church in Ringerike, close to Utoya island, where the shooting spree left at least 85 people dead.
In Shetland, an open air church service brought together hundreds of people taking part in the Tall Ships Race, among them many Norwegians.
The island is only 200 miles away from Norway and enjoys close links with the country. Prayers were said at the service in Lerwick for victims of the Oslo bombing and the Utoya island massacre.
Leading the prayers was Superintendent for the Fisherman's Mission, Aubrey Jamieson.
"There were so many Norwegian sailors here as part of the Tall Ships celebration, so that very much brought it even closer to home," said Jamieson.
"We very much wanted to pray for them this morning to remember those that have been bereaved and those that have been injured as a result of this terrible event."
The man behind both attacks, Anders Behring Breivik, has admitted responsibility and is due to make his first court appearance today. He is suspected to have links with the far right.
Norway is to observe a minute's silence at noon local time.