Ngilu was among five people detained that day during a protest against lawmakers' plans to award themselves bonuses of more than $85,000. She was also beaten up by the police, it is said.
Ngilu minister was arrested late Thursday, but never formally charged. She denounced the police as she left the Criminal Investigations Department headquarters to cheers and applause from dozens of supporters.
"All I can say is that they were out to frustrate me, humiliate me," she said.
Ngilu's attorney said she didn't help the activist escape because police knew she was going to a hospital.
"First of all, she was taking the prisoner to hospital to be treated because the police had beaten her badly," attorney Paul Muite said. The hospital returned Njogu to police custody the next day, so there was no escape, he said.
The five activists were released Thursday after a judge said they had been held without charge beyond the 24-hour limit.
Ngilu was the first Cabinet minister arrested during President Mwai Kibaki's four years in office.
Other activists lauded Ngilu's actions.
"What she did, I think, was honorable and commendable," said Mwalimu Mati, a former head of Transparency International's Kenya chapter. "In the face of an impending violation or an impending felony ... if a minister is standing there, is she expected to watch?"
But police commissioner Hussein Ali said Mrs Ngilu would be taken to court for a yet to be determined offence since detectives were still conducting investigations.
"Police are reviewing her statement alongside evidence gathered from other sources including video, photographic and independent eyewitness accounts, with a view to establishing the criminal offences committed for prosecution in a court of law," Maj Gen Ali said in a statement, an hour after Mrs Ngilu was released.
The minister however said she would not resign from the Cabinet.
She said: "Why should I quit the Government? We must continue fighting for justice. We won't give up our fight for a just society."
Charity Ngilu is generally regarded as a powerful player and a role model in the region for a younger generation of female politicians.
When Ngilu announced in 1997 that she intended to run for Kenya's presidency, excitement rippled across the country. As the first woman presidential candidate in sub-Saharan Africa, Ngilu was a trailblazer on a continent known for its corrupt "Big Men."
Though she didn't win the top job in 1997, Ngilu left her mark on the political landscape. In 2002, when opponents of President Daniel arap Moi joined forces as the National Rainbow Coalition (NARC), she became known as "Mama Rainbow." Quick to recognize her contributions to the party after he won the presidency, NARC leader Mwai Kibaki made her one of the key members of his cabinet.
But at the moment she is involved in an intense power struggle in the coalition.