KATHMANDU (Reuters) - Protesters and police clashed in Kathmandu on Saturday at a demonstration by some 30,000 people led by the Maoist opposition to demand that political parties reach a consensus on a new constitution, in the biggest street march in Nepal in years.

Police lobbed teargas shells and used cane batons to stop demonstrators from advancing to the parliament building, leaving more than a dozen people injured, witnesses said.

Thousands of police in riot gear stood behind concertina wire barricades to guard key government buildings across the capital city known for its temples and surrounding mountains.

Almost a decade has passed since the end of a civil war that pitted Maoist guerillas against the state and killed more than 17,000 people. Nepal's political parties have repeatedly missed deadlines to write the new republic's first constitution.

At the heart of the dispute is disagreement over creating new provinces based on ethnicity, a move backed by the former rebels. They have now walked away from the negotiating table, angry at a government plan to use its parliamentary strength to take the constitution to a vote without consensus.

Maoist chief Prachanda, who goes by his nom de guerre meaning "fierce", urged the ruling alliance to give up plans for a vote or face fresh political pressure. "I am ready to die but will not give up the fight for the rights of the people," Prachanda said to cheering crowds waving red hammer and sickle flags.

The constitution is a major outstanding condition of a 2006 peace deal and is expected to create political stability to boost growth, investment and create jobs for thousands of youths who seek menial work in the Middle East, Malaysia and Korea.

Demonstrators said during Saturday's rally that they would not accept a constitution passed without their agreement.

Anjang Gurung, 25, took an overnight bus from the western tourist town of Pokhara to Kathmandu to participate in the protests. "We need our own state according to our caste and religion," he said.

The government says it will go ahead with the voting if the opposition failed to return to talks, but has not set a deadline. Some analysts think talks will resume soon.

"Both sides need a face saving device. In today's protests the Maoists and the opposition alliance were able to improve their bargaining positions," said Kunda Dixit, editor of newspaper the Nepali Times.