* Opposition says Merkel trying to avoid poll defeat
* Former court chief says plant closures unconstitutional
* Czechs criticise "hasty decisions" on nuclear closures
* Carbon allowance prices near 2-1/2 year high (Adds Merkel, E.ON quotes, updates plant closures)
By David Stamp and Annika Breidthardt
BERLIN, March 17 (Reuters) - Chancellor Angela Merkel said on Thursday she aimed to accelerate Germany's move away from nuclear energy after the crisis in Japan and dismissed accusations she may have closed seven atomic plants illegally.
Merkel this week backtracked on an unpopular decision to extend the life of ageing nuclear stations, drawing scorn from the opposition which says she is merely trying to avoid a major electoral setback in regional polls this month.
Addressing a rowdy session of parliament, Merkel said nuclear technology remained a transitional source of affordable power while renewable energy sources were developed further.
A government decree on Tuesday ordered the closure of all nuclear plants which began operating before 1980 for at least three months, so that they could undergo safety checks.
"We will use the moratorium period, which we deliberately set to be short and ambitious, to drive the change in energy policy and accelerate it wherever possible, as we want to reach the age of renewable energy as quickly as possible," she said.
Imposing the moratorium, Merkel suspended a government decision taken only last autumn to prolong the life of Germany's 17 nuclear power plants beyond their original closure dates.
This week's decision drew criticism at home and abroad. A former president of the Constitutional Court, Hans-Juergen Papier, said the volte face was unconstitutional.
"Constitutionally it goes without saying that the federal government cannot order the provisional repeal of a law," he told Handelsblatt newspaper. Asked if the government's move had been unconstitutional, he said: "Yes, that's how it is."
Amid opposition heckling, Merkel said repeatedly that the catastrophe in Japan, where an earthquake and tsunami provoked a crisis at the Fukushima nuclear complex, meant Germany faced a new situation. Everything had been done legally, she said.
"The nuclear law provides precisely for this: shutting down a plant temporarily until the authorities have achieved clarity about a new situation," she said. "We should not insinuate there are legal tricks where there are none."
An official in the neighbouring Czech Republic also questioned how major decisions could be made when Japanese engineers were still battling to avoid a major nuclear accident.
"We have time to analyse what we can learn from Fukushima," said Dana Drabova, who heads the Czech nuclear safety office.
"But to make hasty decisions when we hardly know what happened in the plant and where improvements should be appears to me rather premature," she told Reuters TV. "Technically, I see no reason for the shutdowns."
Bavaria's Isar 1 reactor went offline on Thursday and the Unterweser plant in Lower Saxony will probably shut down on Friday, their operator E.ON E.ON said. [nLDE72G1HN]
E.ON chief executive Johannes Teyssen criticised the government order: "There is no safety-related reason to take these plants off the grid. I view this purely as politically motivated activity," he said. [nLDE72G0MU}
Those closures are likely to be followed by shutting EnBW's (EBKG.DE) Neckarwestheim 1, a target of anti-nuclear protests in the state of Baden-Wuerttemberg, where Merkel's conservatives face a struggle to hold on to power in a March 27 election.
Defeat there would deliver a psychological blow to Merkel and weaken her coalition, which has already lost its majority in parliament's upper house, where representation depends on the relative strength of parties in state assemblies.
The CDU was thrashed in an election in Hamburg last month and is desperate to hold on to Baden-Wuerttemberg, an economic powerhouse and conservative stronghold for more than 60 years.
Germany will have to raise power production from fossil fuels to compensate for the nuclear closures and the uncertainty over Merkel's plans pushed up the cost of carbon emission permits to a 2-1/2 year high this week. [nLDE72G0MT] (Additional reporting by Stephen Brown, Tom Kaeckenhoff and Vera Eckert; Editing by Peter Graff)