Security personel stand at the site of a blast where a bomb attached to a motorbike exploded killing four protesters in the southern city of Janakpur on April 30, 2012.

Kathmandu -

Nepalese police have arrested a lawmaker for allegedly masterminding a bomb attack that killed four people and injured dozens of others in 2012, an officer told AFP on Monday.

Sanjay Kumar Sah, a former government minister, was arrested late on Saturday over the attack targeting 150 protesters rallying in the southern city of Janakpur, 20 kilometres from the Indian border.

Police accuse Sah of holding a personal grudge against one of the protest leaders, who escaped unharmed when the bomb strapped to a motorbike exploded in April 2012, killing four and injuring nearly 30 others.

“We have strong evidence that Sah planned the attack,” district police official Uttam Subedi said.

A trial against Sah, from the regional Sadbhawana Party, is underway following his arrest in the border town of Birgunj for allegedly plotting the attack.

Police say Sah, a minister from 2009 to 2010, had a grudge against the protest leader after competing with him for lucrative real estate contracts. Both men are accused of using armed gangs to secure development projects.

At least two other people, including Sah's aide, have also been arrested over the blast, which hit protesters campaigning for a separate province for the Maithili-speaking minority in Nepal's southern plains.

“Their statements revealed Sah's role in the attack. We now have phone and transaction records to back their claims,” Subedi told AFP.

Nepal has been relatively peaceful since former rebel Maoists laid down arms after waging a 10-year insurgency and signed a peace accord in 2006, paving the way for the country's first post-war polls two years later.

But since 2008 the Himalayan nation has been politically paralysed, with the constituent assembly - which serves as the parliament - perpetually deadlocked over plans to draft a constitution which will divide the country into new federal states. - Sapa-AFP