A prominent activist, Ahmed Douma, was sentenced on Wednesday to life in prison, officials said, part of a sustained crackdown on Islamist and liberal opponents of the government. Mr. Douma, a leading figure in the pro-democracy revolt that toppled President Hosni Mubarak, was convicted of rioting, inciting violence and attacking security forces in 2011. Mr. Douma is already serving a three-year sentence for taking part in protests. The court also sentenced 229 others to life in prison in absentia for the same charges, and 39 minors received 10-year sentences in absentia. A State Department spokeswoman, Jen Psaki, said that the United States was “deeply troubled” by the sentences.