"With our blood and soul we sacrifice for you, Bahrain," they chanted as they walked, according to videos of a recent protest posted on the internet. Some wore scarves to conceal their faces. Others waved Bahraini flags.
"People are boiling," one Shi'ite activist in Qatif told Reuters by phone, asking not to be named for fear of arrest. "People are talking about strikes, demonstration and prayer to help the Bahrainis."
The protests were in response to Saudi Arabia, the world's top oil exporter and most powerful Gulf Arab state, sending troops to Bahrain last week to help quell weeks of protests by majority Shi'ites in the Sunni-led monarchy. Bahrain's opposition called it a declaration of war.
Riyadh, facing Shi'ite protests of its own, fears a sustained revolt in neighbouring Bahrain could embolden its own Shi'ite minority, which has long grumbled about sectarian discrimination, charges Riyadh denies.
The military intervention, however, appears to have only deepened Shi'ite resentment in the kingdom, where between 10 and 15 percent of the 18 million Saudi nationals are Shi'ites.
Leading Saudi Shi'ite cleric Sheikh Hassan al-Saffar has called for Gulf leaders to find a political solution.
Saudi Shi'ites, inspired by pro-democracy protests across the Arab world that toppled the leaders of Egypt and Tunisia, have held sporadic protests in a handful of eastern towns over the past three weeks.
"Before the start of revolution in Tunis, people felt rather incapable of making a difference," activist Tawfiq al-Seif said. "They (now) feel they can make a difference."
But Saudi Arabia's Bahrain intervention, as part of a Gulf force, has raised the stakes even as a broader protest movement has not taken hold. The scenes at recent protests were in stark contrast to previous demonstrations focused on narrow demands like securing freedom for long-held detainees.
In those early protests, women in black abaya cloaks held out roses and men waved the Saudi national flag, videos showed. "Freedom for prisoners," one sign read. "Justice is our demand," said another.
By day, Qatif is a place of contrasts. Carefully planted white flowers flutter in road medians and luxury villas with decorative stone walls contrast with apartment blocks with exposed cinderblock or faded and peeling paint.
Pictures of King Abdullah abound in the town, the outskirts of which are dotted with date palms. Graffiti, which a government escort says was likely words of romance, has been scribbled over to make it illegible.
"Everything is fine. We are good," said one man, asked about the situation in Qatif on a main street where protests have taken place, speaking as a government escort looked on.
But activists say there is significant resentment, stemming from the feelings of inequality by Shi'ites who want better infrastructure in their towns, improved access to jobs and to be treated as equals in the Sunni-dominated kingdom.