(Adds rebel groups claiming responsibility, revises toll, adds details) By Gopal Sharma

KATHMANDU, Jan 30 (Reuters) - A bomb exploded near a government-sponsored political rally in southern Nepal on Wednesday wounding at least 35 people, police said.

The blast in the town of Birgunj in the southern plains of the Himalayan nation, where thousands had gathered for the rally, followed two earlier explosions in the area including one on Tuesday night.

"We have 55 people wounded in all," a police official told Reuters from Birgunj, 62 km (39 miles) from the capital Kathmandu. The toll included those from the overnight blast as well as from Wednesday.

A doctor at the local hospital said at least three people were in critical condition following the blast at the rally.

"I think it was a plastic bomb," police official Yogeswor Romkhani said.

State-run Radio Nepal said three armed groups fighting for regional autonomy had claimed responsibility for the blast, the latest in a series that have rocked the restive southern Terai plains in recent weeks.

Police could not immediately confirm Radio Nepal's report.

Scores of people have died in ethnic violence and clashes with rebel groups in the region, also known as the Madhesh, in the past year.

"I was standing near the site when I heard a sudden blast," said Jaya Narayan Yadav.

"Then I saw people running away in panic. Some were bleeding from their legs, hands and the back."

Police said the blast took place 200 metres from the political rally which was being addressed by senior leaders of the ruling seven-party alliance.

The rally was the latest in a series of public meetings the ruling alliance had organised this month after setting Apr. 10 for twice-delayed elections for a constituent assembly.

That assembly, a centrepiece of a peace deal with former Maoist rebels, is supposed to prepare a new constitution, declare Nepal a republic and make laws.

Ethnic groups from the Terai, including armed groups, have been demanding greater participation in the assembly or regional autonomy ahead of the vote.

They say the fertile area which is home to nearly half of Nepal 26.4 million people has been discriminated against by the government which is dominated by the people from the hills.

The United Nations, which had its monitors at the rally, condemned the blast.

"Violence and intimidation have no place in a democratic transition, and in particular Nepal's constituent assembly election process," said Ian Martin, chief of the United Nations Mission in Nepal, monitoring the peace deal and elections.

"Political differences must be resolved through peaceful means."

The multi-party interim government says it is ready for talks with the ethnic groups as well as the armed rebels and urges them to participate in the upcoming elections, which will be Nepal's first national vote in nearly nine years. (Editing by Jonathan Allen and Sanjeev Miglani)

AlertNet news is provided by