An increasingly violent new ultra-left rebel group responsible for gunning down at least 20 members of rival organisations is posing the most dangerous challenge to the Marxist government of West Bengal. The extremists have forged a tactical alliance with Maoist guerillas in neighbouring Nepal, and Bengal's jittery communist state government has responded by throwing more than 500 rebels into jails amid widespread allegations of midnight arrests without warrants and torture of suspects, including women, to smash the growing rebel network. But the unprecedented red versus red battle in India's reddest state - which has been under communist rule since 1977 - is baffling analysts. Suddenly, after 25 years in power, Bengal's ruling Left Front - a coalition of the Communist Party of India (Marxist) and nine smaller socialist parties - is now battling an armed, ultra-left shadowy outfit calling itself the Communist Party of India (Marxist-Leninist) People's War, or PW. The PW - formed only last August - has killed 20 Communist Party of India members in broad daylight, snatched hundreds of police weapons and established 'liberated' zones in three poor districts. What is most worrying for the Left Front is the support PW, essentially a rural phenomenon, seems to enjoy among urban intellectuals ranging from writers and poets to teachers, engineers, doctors and even middle-ranking government officials. Last week, a group of prominent intellectuals wrote to Chief Minister Buddhadev Bhattacharya demanding an immediate stop to police excesses against PW sympathisers, while college lecturers demonstrated against human rights abuses. On Saturday, three suspected PW members underwent a hospital examination in Calcutta to verify claims of police torture. The examination was ordered after Calcutta physics lecturer Kaushik Ganguly complained to the judicial magistrate that he was beaten up by police during interrogation for alleged PW connections. The medical report confirmed that injuries were inflicted on Mr Ganguly and two others. The Association for Protection of Democratic Rights has petitioned the West Bengal Human Rights Commission to send doctors to treat scores of victims of torture languishing in jails. PW is led by an elusive 30-year-old strategist codenamed 'Somen', who has apparently never been seen in public or photographed. The kingpin was reportedly inspired by Nepal's Maoists fighting to establish a communist republic. Saying it is opposed to what it calls 'post-colonial imperialism and feudalism', PW has condemned the Left Front as rightist. The group exhorts its followers not to be fooled by the Front's conventional politics, trade union activity or farm workers' agitation but to seize political power. The Front set a national record, winning five successive elections since capturing power, while repeated attempts by right-wing parties such as Congress, and later the resurgent Bharatiya Janata Party and Trinamul Congress, have so far failed to dislodge it. The main reason for the Front's success was its land-reform programme, which established the rights of landless labourers and farmers over the land they tilled, turning the state's rural population of around 65 million into a vote-bank impenetrable by any other political party. And availability of work in villages reduced the pressure on the cities, making political management of urban problems easier. Besides the success in the countryside, the Front's astute handling of popular discontent and the curbing of agitations along caste or religious lines earned Bengal the distinction of being one of India's most peaceful states. But now with radical ideological cousins on the warpath, a complacent Communist Party of India (Marxist) seems to have been jolted out of its stupor.