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HELSINKI, Dec 10 (Reuters) - Finland's parliament approved the nomination on Tuesday of 34-year-old Social Democrat Sanna Marin as the world's youngest serving prime minister.

Out of the parliament's 200 members, 99 voted in favour and 70 against her nomination. Thirty deputies were absent.

"Within four years we won't complete Finland but it can become better... I want to build a society in which every child can become anything and in which every human being can live and grow old with dignity," Marin wrote on Twitter.

Finland's government resigned last week after the Centre Party said it had lost confidence in Social Democrat Prime Minister Antti Rinne over his handling of a postal strike.

After parliament's approval, Finland's president was expected to nominate Marin's five-party coalition cabinet, consisting of 12 female and 7 male ministers.

But behind the festivities, deep divisions remained between the main coalition partners, Marin's Social Democrats and the Centre Party.

Marin will struggle to defend her leftist views against the Centre Party, which wants action to boost Finnish employment to pay for the costly welfare state. Strikes continued as she took office.

Katri Kulmuni, chairwoman of the Centre Party which holds the balance of power in parliament, defended her decision to oust the outgoing prime minister Antti Rinne, accusing him of taking the employees' side in recent labour market disputes.

Before his resignation, Rinne defended the publicly-owned postal service's employees in their labour dispute by saying their employment conditions would not be trampled while his government was in office.

"It became sort of an habit to flag in advance in favour of one side, in matters which should be dealt with cool impartiality," Kulmuni wrote in a long post on Facebook.

(Reporting by Anne Kauranen; Editing by Jon Boyle and Andrew Heavens)

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