Three arrested after London protest becomes unruly ; officer knocked out . Police say 20,000 gathered outside Israeli Embassy in London . Similar demonstrations take place in other European cities . - The U.S. State Department says it is "gravely concerned" about the situation and will be sending a team to monitor developments on the ground.
Three arrested after London protest becomes unruly ; officer knocked out . Police say 20,000 gathered outside Israeli Embassy in London . Similar demonstrations take place in other European cities . 3,000 protest outside temporary residence of U.S. President-elect Obama .