The churchgoers were injured and taken to a nearby hospital, where two were in a serious condition, said Rose Mureshyankwano.
She said 17 of those injured were still in hospital while the rest have been discharged.
A similar incident happened on Friday when lightning struck a group of 18 students, killing one.
Last October, lightning killed 18 people in different parts of the country.
Lightning strikes are frequent across Rwanda, which has many hills and mountains, and the police record a number of human and livestock deaths each year.
Lightning killed 30 people, injured 61 and killed 48 livestock in 2016, the most recent year with full figures, according to Rwanda's ministry of disaster management and refugee affairs.
Reporting by Associated Press via the Press Association
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