SANA, Yemen  Young protesters in Yemen squared off against security forces on Sunday, and some marched on the presidential palace here, witnesses said, as a third day of demonstrations sought to emulate the revolution in Egypt.

The protests, organized largely via text message, were the largest yet by young Yemenis, with more than 1,000 marching. And it appeared to mark a rift with opposition groups who had organized previous demonstrations that wrested significant concessions from President Ali Abdullah Saleh, including the promise that he would relinquish power in 2013.

Those established opposition groups did not join the crowd on Sunday, which was calling for the immediate ouster of the president. After the initial demonstration, a smaller group of young protesters peeled off and marched toward the presidential palace, only to be violently repulsed by armed security forces both uniformed and in plain clothes, some armed with stun guns, witnesses said. There were reports of several injuries, but no deaths.

“The J.M.P. in our opinion  the opinion of the students  is that they move in stages,” said a 30-year-old protester, Mohamed Mohsin, referring to the Joint Meeting Parties, a coalition of opposition parties. “But we go to the demonstrations to send the message to the leadership now.”