He said the prayers and sermons at Imam Hussein Mosque in the Eastern Province town of Awamiya drew Shiites from nearby Qatif and the Al-Ahsa region.
"They gathered to demand their freedom," said the resident who asked for anonymity.
Their call came as the family of one Shiite activist, sentenced to death after protesting, voiced concern over his fate and a rights group warned of his imminent execution.
Ali al-Nimr's uncle told AFP on Friday his family fears "the government is serious, very serious" about carrying out the sentence.
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Jaffar al-Nimr said his nephew reported that he had undergone a medical check when his family last spoke to him two weeks ago.
Amnesty International warned Thursday that Ali al-Nimr was among more than 50 people, including other Shiite activists, at increased risk of soon being put to death in a single day.
Ali al-Nimr's case has provoked a call for clemency from France, while the United States expressed "deep concern".
Activists say Ali al-Nimr is one of three Shiite protesters who were minors at the time of their arrest and have been sentenced to death. They have exhausted all appeals.
"International law prohibits the use of the death penalty against anyone under the age of 18," Amnesty said.
It added that there were "credible allegations" the three youths had been tortured.