The head of the underground Catholic Church in Shanghai, Bishop Joseph Fan Zhongliang, has died, a US-based group said, as it urged China's Communist government to allow greater religious freedom. Fan died at home on Sunday after several days of high fever, said the US-based Cardinal Kung Foundation, which seeks to promote the Roman Catholic Church in China. It gave his age as 97 though other sources said he was 96. He was held under house arrest for much of the last two decades, the group said. China has a state-controlled Catholic Church as well as an "underground" church, though worshippers typically move between the two. The Cardinal Kung Foundation said authorities denied a request by worshippers to use Shanghai's main Catholic cathedral for Fan's funeral service. A photo posted on social media showed his coffin at a local funeral home with a priest conducting a mass. "A true giant of faith! He fought this battle for happiness his whole life," said one online tribute. Experts estimate that there are as many as 12 million Catholics in China, split roughly evenly between the two churches. China's Communist regime broke ties with the Vatican in 1951, and although relations have improved in recent years as the country's Catholic population has grown, they remain at odds over which side has the authority to ordain priests. Shanghai is considered an important diocese given the city's historical ties to the Catholic Church -- it was home to Xu Guangqi, one of the most prominent converts secured by 16th-century Italian missionary Matteo Ricci. The long-serving bishop of Shanghai's state-run Catholic Church, Aloysius Jin Luxian, died last year at age 96. His appointed successor, Thaddeus Ma Daqin, was stripped of his title of auxiliary bishop after he dramatically split with official religious authorities, denouncing them at his own installation ceremony last July. He is believed to have been confined since then at a seminary on the outskirts of Shanghai. "As both Bishop Aloysius Jin Luxian and Bishop Joseph Fan Zhongliang are deceased, the Cardinal Kung Foundation appeals to the Chinese government to release Bishop Ma immediately so that the Catholic Diocese of Shanghai will have a bishop to lead their ministry," the group said in a statement. "By reinstating Bishop Ma to his rightful office, China will be taking an important step forward in honouring religious freedom."