The pop superstar, 39, revealed the legal arrangement that controls her life is preventing her from marrying boyfriend Sam Asghari and starting a family.
Ms Spears told the court it requires her to use a birth control device to prevent her getting pregnant.
She called for the conservatorship to be ended without the need for a medical evaluation.
Following her dramatic appearance, celebrities publicly shared their backing for Spears.
Mariah Carey tweeted: “We love you Britney!!! Stay strong.”
Pop star Halsey said she “admires” Spears’ courage for speaking out, writing online: “Bless Britney and I hope with my whole heart she is awarded freedom from this abusive system. She deserves it more than anything. I admire her courage speaking up for herself today.
“Additionally, f*** anyone who thinks they have the authority as an institution or individual to control a person’s reproductive health. #FreeBritney.”
Justin Timberlake, who had a high-profile relationship with Spears in the late 1990s and early 2000s, said what is happening to his former girlfriend is “just not right”.
The actor and singer, 40, said: “After what we saw today, we should all be supporting Britney at this time. Regardless of our past, good and bad, and no matter how long ago it was… what’s happening to her is just not right.
"No woman should ever be restricted from making decisions about her own body.
“No one should EVER be held against their will… or ever have to ask permission to access everything they’ve worked so hard for.”
Reality TV star Khloe Kardashian said: “No one should be treated like this. Stay strong Queen!! You deserve better #FreeBritney.”
Meghan McCain, a TV personality and daughter of former US presidential nominee John McCain, suggested the terms of Spears’s conservatorship should be illegal.
“How is what has been done to Britney Spears not a human rights crime?” she said.
“Isolation, controlling her capacity to reproduce without her consent, forcing her to work under inhumane conditions.
"If she were any other person the people who did this to her would be in jail.”