The demonstrators are demanding democratic reforms in the country. Earlier demonstrations were put down with much force resulting in many dead and wounded. Many protesters were imprisoned including doctors who treated the wounded.
Protesters tried to reach the famous Pearl Roundabout but security forces blocked the roads, Protesters threw rocks at the police who responded with tear gas.
The GCC (Gulf Cooperation Council) sent troops many of them from Saudi Arabia to help control earlier demonstrations. These demonstrations went off without any significant casualties it would seem and was mostly peaceful. The regime has been somewhat more restrained about repressing demonstrations of late.
The Bahraini monarchy has for a long time been a staunch U.S. ally. Not only is it headquarters for the fifth fleet but its Sunni dominated government helps to restrain Iranian influence in the area.
Despite a report that claimed the regime used systematic torture during a period of repression against protesters the U.S. has shipped military equipment to Bahrain. The Bahraini government claims to have instituted some reforms. The monarchy blames Iran for the unrest. I have appended a video of the recent demonstrations--from Iranian TV of course! For more see this article and here.