KAMPALA (Reuters) - A law amending Uganda s constitution to allow aging leader Yoweri Museveni to extend his rule was introduced in parliament on Tuesday, at a session where nearly all its opponents were either barred or stayed away in protest.    Museveni, 73, has ruled Uganda since 1986. He is as yet ineligible to seek re-election in the next polls in 2021 because the existing constitution places an age ceiling of 75 on anyone aspiring to the presidency.  The bill brought to parliament would remove the age hurdle. Parliament spokesman Chris Obore said the bill was read and then referred to a parliamentary committee.  The committee will scrutinize it, hold public hearings and make a report on it ... then it will be returned to the House,  he said.  A broad spectrum of Ugandans including the political opposition, religious leaders, rights activists and some members of the ruling party have voiced opposition to removing the age cap.  Last week, fighting broke out in parliament for two consecutive days between legislators opposed to the plan and those supporting it.  On the second day of the fighting, House Speaker Rebecca Kadaga suspended 25 MPs who oppose the law for three sessions for unruly conduct. Security personnel, who MPs say were soldiers in plain clothes, were brought in to eject them and some MPs were injured in the process. Dozens of other MPs from both the opposition and ruling party opposed to removing the age cap walked out of the House in solidarity and have vowed not to return for the duration of their colleagues  suspension.  When the bill is brought back to the House, Obore said, parliamentarians will debate the committee s findings, make their own recommendations and take a final vote. Museveni s bid to remain in power in Uganda, a prospective crude oil producer and Africa s largest coffee exporter, follows a trend in the region.  In Burundi and the Democratic Republic of Congo efforts by leaders to change constitutional rules to retain power have sparked instability. 