The feminist academic and writer Stella Nyanzi has been released from prison after her 18-month sentence for insulting Uganda’s president was quashed.

Nyanzi collapsed as she left court in Kampala on Thursday, and scuffles broke out between her supporters and prison wardens, who fired live rounds into the air to disperse the crowd.

Nyanzi was found guilty of cyber harassment of the president last year, after writing a poem about the president’s mother’s vagina.

But high court judge Henry Peter Adonyo said Nyanzi’s right to a fair trial was violated because magistrates denied her the right to identify, prepare and call defence witnesses.

“The lower court didn’t have the jurisdiction to hear the case. The appellant should be released immediately,” he said.

The judge also dismissed the state appeal on Nyanzi’s acquittal on charges of offensive communication.

Her supporters packed the courtroom in the capital, Kampala, and cheered at the verdict.

After the ruling, Nyanzi said: “Why was I in court for all these months? Why is the current regime of Uganda oppressing Ugandans who are expressing their constitutional rights? I am the voice for the opposition of Uganda.

“Museveni must go. Yoweri Museveni you are on notice. I give you notice, Museveni. You can do whatever you want. We are ready for you, Museveni. We are tired. Stop oppressing Ugandans,” she said.

“It’s important for us the opposition to find bases of unity that are going to help us in our solidarity against the current regime.”

The journalist turned researcher added: “Why was I in prison because I wrote a poem? Because I expressed my deep disinterests and disgust of the NRM [National Resistance Movement] regime?

“Is it because I told the current illegal president of Uganda that I really want him to go? Museveni is sending so many opposition activists to prisons – for what?” asked Nyanzi, who has been held in Luzira women’s prison.

In 2017, Nyanzi spent 33 days in prison for a Facebook post in which she described Museveni as a “pair of buttocks”, after the president backtracked on a pledge to distribute sanitary towels to schoolgirls.

She previously branded the first lady, who is also education minister, “empty-brained”. She was acquitted of a charge of offensive communication.

“Throughout the trial, she [Nyanzi] has consistently voiced the fight will not be won in courts. Her struggle is for equality and [the] end of dictatorship in Uganda,” said Rosebell Kagumire, a feminist activist.

Mwafrika Mbarikiwa Nana, a human rights defender and friend, said a weekend-long welcome home party was being organised.

“Her release is of course encouraging,” said Nana. “It puts [the] courts of Uganda on trial. As women we are very encouraged that a woman could stand to challenge power, challenge places of power.

“It’s a big win for activists, it’s a big win for women and it’s a big win for Ugandans.”

In a statement, the Clooney Foundation for Justice said it welcomed the release of Nyanzi, who it she had been unfairly imprisoned. “After a blatantly unfair trial that was given a ‘D’ by the TrialWatch experts that monitored and graded the trial, the appeals court in Uganda threw out the conviction partly on the basis that the defence team did not have adequate time to prepare a defence. This finding echoes the TrialWatch Fairness Report, which concluded that the trial court ‘violated Dr. Nyanzi’s right to call and examine witnesses and, correspondingly, her right to adequate time and facilities for the preparation of a defence’.”

Nyanzi, a mother of three who was dismissed from her research job at Makerere University, was last month awarded the Oxfam Novib/PEN International award for freedom of expression.