Brazzaville - Congo Republic's opposition parties have convened an Alternative National Forum in rejection of a government-sponsored conference that opens the door to President Denis Sassou Nguesso seeking a third term next year.The government-backed talks this month, boycotted by the main opposition alliance FROCAD, recommended changing the constitution to remove a two-term limit on the president and to scrap an age cap of 70, both of which would have disqualified the 71-year-old Sassou Nguesso from running again.The former military officer has ruled the oil-producing central African nation for a total of 31 years in two spells since 1979."Altering the January 20 2002, constitution would be a constitutional coup because it would violate the spirit, the principles and the respect of that constitution," Guy Romain Kinfoussia, a senior member of FROCAD, said in an opening address to the conference late on Sunday.Congo is on track to leapfrog Equatorial Guinea to become sub-Saharan Africa's third-largest crude producer by 2017, yet half of its 4.5 million population live in poverty, according to the World Bank.The opposition forum, grouping some 650 delegates and 15 opposition groups, is due to run until Wednesday in the capital Brazzaville.Sassou Nguesso has not spoken publicly about whether he intends to run next year but his ruling Congolese Labour Party (PCT) has urged him to do so. Ahead of this month's conference, he said he would put any constitutional changes to a referendum.Opposition to veteran leaders extending their terms has triggered protests in several African countries in recent months.Burkina Faso's president, Blaise Compaore, was toppled by demonstrations in October, while Burundi's Pierre Nkurunziza overcame popular resistance to win re-election to a third term as president last week, in polls that Washington branded "deeply flawed"."Congo is living a dangerous period," said Jean Itadi, a member of the main opposition party, the Pan-African Union for Social Democracy (UPADS). "Since President Sassou came to power, we have not had a free and transparent election in this country."