A senior politician in India’s ruling Congress Party has been killed in an ambush by Maoist insurgents in which 25 people died.

An estimated 300 guerrillas known as Naxalites attacked a convoy of senior Congress figures on Saturday afternoon as they returned from a rally in Chhattisgarh, one of India’s poorest states and the centre of a Maoist insurgency.

As the heavily guarded convoy of about 40 cars drove through a densely forested area, the attackers blocked the road by felling trees. They detonated a landmine and raked the vehicles with gunfire from the surrounding high ground for nearly an hour and a half.

The dead included the local Congress leaders Mahendra Karma and former national cabinet minister VC Shukla.

Nand Kumar Patel, the Congress party state leader for Chhattisgarh, was initially reported missing and presumed kidnapped. But he and his son were found dead yesterday, close to the scene.

“When our cars reached a turning point, the Naxals started firing,” an injured Congress worker told NDTV.

“Two cars were blown up and the firing continued for almost one and a half hours. Many people were killed and many sustained bullet injuries. Some of us lay on the road to save ourselves.”

The scale of the assault on a convoy protected by the highest security India offers caused shock across the political spectrum. It was denounced as an attack on democracy by both Congress and Bharatiya Janata Party leaders. Narendra Modi, the nationalist chief minister of Gujarat and contender to be his party’s election campaign leader, called for national unity in the fight against terrorism.

“The need of the hour is to stand together as a nation and vow to fight this menace that threatens our democracy,” he said. “The time has come to adopt a policy of zero tolerance towards terrorism.”

Dr. Manmohan Singh, India’s prime minister, and Sonia Gandhi, the Congress Party president, visited the injured on Sunday and denounced the ambush as a “cowardly” attack on India’s democratic values.

The Maoist rebellion in India’s south-eastern states has been gathering pace since it began in Naxalbari in 1967.

It claims to represent poor and downtrodden tribal groups that have suffered land seizures and feel marginalised in India’s feudal hinterlands.

Dr. Singh regards the insurgency as the greatest terrorist threat to India, especially in Chhattisgarh, Orissa and parts of West Bengal where it has some support. In April 2010, Naxalites ambushed and killed 76 paramilitary policemen in Chhattisgarh.

The central government has struggled to contain the insurgency in Chhattisgarh, where local Congress leaders formed the controversial Salva Judum vigilante movement to arm villagers to resist the Naxalites. One of those killed in the ambush on Saturday was Mahendra Karma, the Salva Judum movement’s founder.

At least five policemen also died in the attack in the Jagdalpur area of Bastar district, 233 miles south of Raipur.