Two men, aged 19 and 20, have been arrested and detained on suspicion of robbery, assault and possession of offensive weapons.
Shortly before 7.30pm, a 44-year-old man was attacked near the bandstand in Montague Street, suffering cuts and bruising which required hospital treatment.
At around 9pm, a 25-year-old man was threatened with a chain while walking past McDonalds restaurant in the same street.
And at 9.30pm, a 15-year-old girl was assaulted by two men at the junction of Shelley Road and Portland Road. One man struck her in the face before stealing her iPhone.
Detectives would like to hear from anyone who may have witnessed any of the incidents.
Anyone who saw any of the incidents or who has other information is asked to phone Sussex Police on 101 or email [email protected], quoting serial 1550 of 22/4. Alternatively, they can contact the independent charity Crimestoppers free and anonymously on 0800 555 111.