The opposition parties were projected to win a combined 88 of the 169 seats in parliament, against 81 seats for Prime Minister Kjell Magne Bondevik and his allies.
The leftist coalition, comprising Labour, the Socialist Left Party and the agrarian Centre Party, thereby appeared to have won the absolute majority they have said they needed to form a new government.
"We promised an absolute majority and that's what we're going to give the country," Labour leader Jens Stoltenberg told reporters, adding however that he was still waiting for the final results before claiming victory.
Prime Minister Bondevik conceded defeat after 96% of the votes had been counted. "Politically I am disappointed that (the coalition government) has not been allowed to continue," he said.
The makeup of the next government will not be announced until after parliament re-assembles on 10 October.
An estimated 75.1% of Norway's 3.4 million eligible voters had cast their votes to determine who will lead their country for the next four years.
Opinion polls in recent days have shown Bondevik's minority coalition in a neck-and-neck race with the rival left coalition, as the election campaign focused on the distribution of Norway's abundant oil wealth.
Bondevik has touted the country's robust economic health during his government's four years in power.
The leftist opposition, which has presented a united front for the first time, has meanwhile attacked Bondevik's tax cuts for the wealthy, and proposed instead more spending on education, healthcare and welfare.
The centre-right government has countered that Stoltenberg's promises would destroy jobs and drive interest rates higher.
Minority governments have been the rule in Norway, and prime ministers have enjoyed an outright parliamentary majority only twice in the past 36 years.
This time however, the leftist coalition is already celebrating what it hopes will be an absolute majority.
The victorious opposition will, however, be facing towering challenges as it attempts to trace out policies that all three parties can sign onto.
While the three parties agree on many domestic policy goals, like a more equal division of Norway's wealth, they hardly see eye-to-eye when it comes to foreign policies.