Rwigara together with her mother Adeline Mukangemanyi have been under incarceration over charges of inciting insurrection and forgery as allegedly discovered by the National Electoral Commission during the evaluation of nominations for the presidential elections held previous year.
Ms Rwigara already spent one year in prison after being barred from running in presidential elections against the long-standing incumbent Paul Kagame. A three-judge panel described the charges as "baseless". I am continuing with my political journey ...everything I talked about in the past has not been resolved.
She added: "This is what happens when you dare to have a different political opinion - a different view from those in the government".
Rwigara's father, a prominent businessman who was once close to the ruling Rwandan Patriotic Front (RFP), was killed in a road accident in 2015.
Their release came a month after the unexpected freeing of Victoire Ingabire, another woman who sought to run for the presidency in 2010.
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Paul Kagame, the country's towering, beanpole-like president, has been widely praised for his role in providing stability and economic growth after Rwanda's catastrophic genocide in 1994. "There are still many political prisoners in the country", she told journalists after the high court ruling.
Rwigara says her incarceration was politically motivated because of her decision to vie against Kagame at elections held in August 2017, an allegation authorities deny.
Four other accused relatives, who are not in the country, were also cleared of all charges and found not guilty.
In Thursday's ruling, the high court judges said the prosecution failed to prove that Ms Rwigara had personally forged supporters' signatures and ruled that her criticism of the government during press conferences was protected by freedom of speech in both the constitution and worldwide law. Human rights organisations welcomed this, saying it showed that "Rwanda may be turning a new leaf", but said the arrests, executions, disappearances and torture would have to end to prove that the change was meaningful.
Since her arrest, Rwigara's brothers and sister have been interrogated, family assets have forcibly been auctioned to pay off a multi-million dollar tax claim, while a hotel they owned was demolished for allegedly failing to abide by city guidelines.