The activists met in front of the clinic where Tymoshenko is being treated for back pain.
Supporters from different parts of Ukraine brought postcards and signs saying "Happy birthday, Yulia!"
Police intervened after a group of anti-Tymoshenko activists scuffled with her supporters.
Police took away at least one anti-Tymoshenko activist.
Tymoshenko, a bitter rival of President Viktor Yanukovych, is serving a seven-year prison term for abuse of office.
She and her supporters say the charges were politically motivated.
On November 26, the authorities granted permission to Tymoshenko's daughter Yevhenia to stay with her mother for three days.