MEXICO CITY, June 26 (Reuters) - Nicaraguan police have arrested the brother of detained opposition leader Cristiana Chamorro, accusing him of damaging the country"s sovereignty and continuing the sweeping crackdown of political and business figures opposed to President Daniel Ortega. About 20 prominent Nicaraguans, including Cristiana Chamorro and four other politicians who were planning to challenge Ortega in the November presidential elections, have been arrested in recent weeks. Many have fled abroad. Police late on Friday said Pedro Joaquin Chamorro Barrios was detained under the country"s recently introduced sovereignty law, and was "being investigated for carrying out acts that damage the independence, sovereignty and self-determination" of the Central American nation. A former lawmaker, Pedro Joaquin had been a vocal critic of Ortega and was a member of the opposition Citizen Alliance grouping that was looking to contest the elections. Pedro Joaquin"s brother, prominent journalist Carlos Fernando Chamorro, said on Twitter his brother has been "kidnapped" by Ortega"s police who "fabricate alleged crimes and criminal offenses against citizens who demand free elections". Carlos Fernando had earlier this week fled abroad into exile with his wife after the police raided his home. The Chamorro clan are one of Nicaragua"s most storied dynasties and in the past some family members were allied with Ortega, a former rebel commander who first ruled Nicaragua in the 1980s after his leftist Sandinista fighters swept to power in 1979 by toppling dictator Anastasio Somoza. But over time they all broke off from Ortega and Violeta Chamorro - the mother of Cristiana, Pedro Joaquin and Carlos Fernando - ended Ortega"s first stint in power when she won the 1990 elections to become the president. Ortega, who has ruled Nicaragua since returning to power in 2007, earlier this week justified his crackdown on opponents by saying his administration was arresting and prosecuting criminals who were plotting a coup against him. read more "It"s absurd to set them free. Everything we"re doing, we"re doing it by the book," he said. Ortega"s opponents accuse him of returning the country to the dark days of the Somoza dictatorship.