* Head of Italy's biggest union urges resistance
* Berlusconi calls on opposition to support plan
* Doubts emerge over plan's viability
ROME, Oct 27 (Reuters) - Italy's biggest trade union pledged on Thursday to fight economic reforms that Prime Minister Silvio Berlusconi presented to euro zone leaders to ease a debt crisis, deepening doubts over whether the ambitious plan can be implemented.
Susanna Camusso, secretary of the powerful CGIL union, called on other worker federations to close ranks against the plan, which includes a commitment to raise the retirement age and steps to make it easier for firms to lay off staff.
"We're ready to propose unified action," Camusso said in an interview with the left-leaning La Repubblica daily, describing the reforms as a combination of "targeted attacks" on Italian workers.
Berlusconi submitted a hastily constructed package of reforms to a summit of European leaders on Wednesday in response to an ultimatum demanding action to boost growth and cut Italy's huge public debt.
After the summit, Berlusconi appealed to opposition leaders to support the plan, saying it was in Italian interests.
"We hope the opposition wants to get out of this situation of always saying no and always being against things, and that it will back approval of these measures requested by Europe," Berlusconi told reporters.
But several commentators raised doubts about the viability of the plan on Thursday, pointing to tensions within the ruling coalition and a poisonous political climate in Rome.
"It's difficult to believe that yesterday's intentions can really be transformed into the biggest plan of market reforms Italy has ever put on paper," Antonio Polito wrote in the Corriere della Sera daily.
In an editorial in La Repubblica, Massimo Giannini described the plan as a "book of dreams" which would not solve Italy's economic problems.
Berlusconi carried a "letter of intent" to the EU summit, promising a much delayed economic development plan by Nov. 15 and a series of other measures to increase growth and ensure the budget is balanced by 2013.
They formed part of a euro zone deal with private banks and insurers aimed at drawing a line under spiraling debt problems that have threatened to unravel the European single currency project.
After dodging the worst of the financial crisis in recent years, Italy has moved to the centre of the euro zone debt crunch this year as its bond yields soared to near unsustainable levels. Only intervention from the European Central Bank has prevented them from sliding out of control.