The attack came after thousands of Saudi protesters took to the streets in Qatif on Friday, demanding the release of political prisoners, including a prominent Shia cleric.
Many demonstrators were also arrested in the crackdown.
On Thursday, thousands of protesters took to the streets in the city, calling for the release of political prisoners and the downfall of the US-backed monarchy.
Tensions are running high in Saudi Arabia's oil-rich Eastern Province in the past weeks following the detention of Shia cleric Sheikh Nemr al-Nemr.
Sheikh Nemr was attacked, injured and arrested by Saudi security forces while driving from a farm to his house in the Qatif region of Eastern Province on July 8.
His family members said he has been badly tortured in jail after they were allowed to visit him in prison. Sheikh Nemr is on hunger strike since July 19.