The clash took place in the city of Cherkasy, about 150 kilometers (90 miles) southeast of the capital Kiev, according to a police statement.
Several dozen members of the National Corps party tried to rush onto the stage where Poroshenko was speaking and tried to block his motorcade. Police fired tear gas to disperse them. The statement did not state the severity of police injuries but said five of the officers were treated at the scene.
Poroshenko was in Cherkasy campaigning for a new term. Ukraine holds presidential elections on March 31.
Earlier Saturday, police officers were injured in a clash with far-right demonstrators in Kiev.
The violence occurred outside the presidential administration building in Kiev where several hundred demonstrators had gathered to call for arrests of top figures in an alleged military corruption scandal.
A media investigation last week detailed alleged embezzlement schemes in Ukraine’s military industry, including at a factory controlled by Poroshenko.
A police statement said the demonstrators tried to break through police lines and were setting off fireworks. Police turned them back with tear gas.
The police said one officer was hospitalized with chemical burns to his eyes.