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.
Egypt court overturns life sentence against ex-president Mursi
Abu Ismail is already serving a seven-year prison sentence after he was convicted of lying in nomination papers for the 2012 presidential elections.
A firebrand fundamentalist leader, Abu Ismail had a devoted following in the run-up to the vote but he was disqualified.
He had claimed neither of his parents were dual citizens but his mother had acquired US nationality.
The election was won by the Muslim Brotherhood's Mohamed Morsi, who became the country's first elected civilian president.
Egyptian judge facing corruption charge hangs himself
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.
Abu Ismail was arrested within days of Morsi's overthrow along with other Islamist leaders including the ousted president.
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