BEIRUT, Lebanon — Yemen, one of the Middle East’s most impoverished countries and a haven for Al Qaeda militants, became the latest Arab state to witness mass protests on Thursday, as thousands of Yemenis took to the streets in the capital and other regions to demand a change in government.

The scenes broadcast across the Arab world were reminiscent of demonstrations in Egypt this week and the month of protests that brought down the government in Tunisia. But as they climaxed by midday, the marches appeared to be carefully organized and mostly peaceful, though there were reports of arrests by security forces. Predictably, the protests were most aggressive in the restive south.

In Sana, at least 10,000 protesters led by opposition members and youth activists gathered at Sana University, and around 6,000 more gathered elsewhere, participants, lawmakers and activists reached by telephone said. Many carried pink banners and wore pink headbands.

The color was both a unifying symbol and an indication of the level of planning underlying the protests. Weeks ago, as the Tunisian protests were still escalating, a committee from an umbrella group of six opposition parties settled on an escalating scale of color to accompany their own plan of action, starting with purple for lawmakers to show their opposition and moving to pink for the street protests. Red, said Shawki al-Qadi, a lawmaker and opposition figure, would be the final color, though he said the opposition had not yet decided what actions would correspond with the move.