In 1848, with the escalation of repression by the country's Conservative government, Lastarria joined the , a revolutionary group which sought to overthrow Bulnes and the Constitution of 1833. In 1850 he was arrested by the government and sent to Lima. He returned to participate in the Revolution of 1851, seeking to annul the election of Manuel Montt. This was defeated by aggressive government action, and Lastarria escaped back to Peru, labeled one of the "ten most wanted men in Chile". His brother Manuel was arrested by the government.