Supporters of Hazem Salah Abu Ismail used force, violence and threats against prosecutors to try to force them to order the release of a defendant, according to the indictment.
Five other defendants received the same jail terms, while another 13 were sentenced to 10 years in prison in absentia, the judge said in a televised verdict.
Abu Ismail is already serving a seven-year prison sentence after he was convicted of lying in nomination papers for the 2012 presidential elections.
He had claimed neither of his parents were dual citizens but his mother had acquired US nationality.
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The election was won by another Islamist, the Muslim Brotherhood's Mohamed Morsi, who became the country's first elected civilian president.
Morsi ruled for only a year before then-army chief Abdel Fattah al-Sisi led his overthrow amid mass protests against his presidency and the Muslim Brotherhood in July 2013.
Morsi's ouster unleashed a deadly crackdown on Islamists and Sisi was elected president the following year.
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