July 26 (Reuters) - With a seventh opposition presidential hopeful under arrest, Nicaraguan President Daniel Ortega in a speech described his political opponents as pawns being used by the United States against him. Scores of prominent Nicaraguans, including seven politicians aiming to run for president in November, have been arrested in amid accusations of plots to stage a coup. Many have fled abroad. In a speech at a public event on Sunday, Ortega criticized opposition figures saying they were part of the "Yankee policy" and that the opposition "has no shame. They don"t deserve to even be called Nicaraguans ... The empire uses them and when they win, kicks them out, too." Ortega repeatedly referred to the United States as "the empire" and said there was no room for understanding or negotiating with Washington. The United States has called the detentions an attack on democracy and imposed sanctions in June against four Nicaraguans, including Ortega"s daughter. read more Ortega, who is seeking a fourth consecutive term, said: "The empire does not want elections in Nicaragua, the empire wants to boycott them, it wants to re-sow terrorism in our country." The U.S. State Department earlier in July also imposed visa restrictions on 100 Nicaraguans affiliated with the Nicaraguan National Assembly and judicial system. read more Nicaragua"s national police said they detained the seventh opposition figure over the weekend, presidential hopeful Noel Vidaurre. Vidaurre was placed under house arrest for "carrying out actions that undermine independence and incite foreign interference in internal affairs," among other accusations, according to the police statement.