Kinshasa - Security forces fired tear gas at hundreds of rock-throwing protesters in Democratic Republic of Congo's capital on Monday as opposition parties try to block election reforms that may delay elections due in 2016.Smoke billowed into the air as tyres burned in the streets of Kinshasa, where police in riot gear and armed presidential guards were deployed. Two military helicopters flew overhead. There were also demonstrations in Goma, the main city in eastern Congo.The protests are against a revised election law that requires a national census be carried out before elections, a move that could delay the polls by years and allow President Joseph Kabila to put off standing down.The bill was approved over the weekend by the lower house of parliament and was due to be examined by the senate on Monday.Opposition parties said their leaders, who had called on followers to occupy parliament, were unable to join the protests as their offices were surrounded by security forces.Critics call the reform a "constitutional coup" but the government says the census is a necessary part of the electoral process in the vast, mineral-rich country of 65 million people.Kabila's rivals have so far struggled to mobilise large groups, partly because of fears of heavy-handed police tactics, and crowds have in the past been easily dispersed. Ahead of Monday's march, opposition leaders called on supporters to show more resistance and to fight back against police.Police fired tear gas on a crowd of more than 1 000 people in the eastern city of Goma on Monday. A Reuters reporter saw at least two people with bullet wounds in the streets.