Hundreds of thousands of youths demonstrated in cities across the country Saturday in opposition to an agreement last week that grants President Ali Abdullah Saleh immunity from prosecution in return for leaving office. One civilian was killed by government forces on the outskirts of the capital, eyewitnesses said.

"Martyrs wrote with their blood, that Saleh must stand trial," read a banner held up by protesters in the capital.

Yemen has seen some of the biggest demonstrations of the year in recent days, after Mr. Saleh signed the agreement following months of delay. The agreement, negotiated by neighboring Arab countries ...