Luanda - Rights groups accused Angola's government on Wednesday of using the legal system to crack down on critics, after an activist was jailed for six years on charges of stirring rebellion.Jose Marcos Mavungo was found guilty on Monday of an "attack on the sovereignty of the Angolan state" as prosecutors accused him of trying to encourage his restive oil-producing home region of Cabinda to fight for independence."It was a political judgment ... The trial is a serious violation of the basic principles of the democratic state of law," Sizaltina Cutaia, a rights activist at the Open Society in Angola, told Reuters.Amnesty International called the conviction a "blatant violation of freedom of expression". Mavungo, who had denied the charge, is the latest in a string of activists and protestors who have been jailed in recent months.There was no immediate response from the government of Africa's second largest crude exporter.Its permanent representative to the UN said on Monday it respected human rights. "Those who seek to underestimate the progress made in this field in Angola, they do so in bad faith," Apolinario Correia added in a statement.Mavungo was arrested in March 14, the day he planned to lead a protest in Cabinda against "poor governance and human rights violations". He has been a supporter of a peaceful campaign for the independence of the northern exclave."Ridiculous sentence"At the trial he was accused of being linked to men found with explosives on the day of the rally, which was eventually cancelled."There was no proof of this act of rebellion... It is a ridiculous sentence, pathetic and meaningless," defence lawyer Luis dos Nascimento told Reuters.Angola has worked to rebuild its reputation since a 27-year civil war ended in 2002 and it is now China's biggest trading partner in Africa and the second largest for the United States.Painful memories of the war and the best funded security forces in Africa have helped keep the peace, though there have been growing signs of public anti-government sentiment.A halving of oil prices last year has piled hardships on Angolans as the kwanza currency plummeted and government slashed public spending in one of the most unequal societies in the world.Cabinda has suffered sporadic unrest during a four-decades-long battle for independence from Angola by rebels from The Front for the Liberation of the Enclave of Cabinda.Oil output from wells off the coast of Cabinda, which is sandwiched between the Democratic Republic of Congo and the Republic of Congo, represents more than half of the around 1.8 million barrels per day produced by Angola.