JERUSALEM - Israel tightened security in Jerusalem on Friday, anticipating Palestinian violence in reaction to its sea raid on a Gaza-bound aid ship earlier this week, an Israeli police spokesman said.

US President Barack Obama said the incident at sea in which nine international pro-Palestinian activists were killed while trying to break the Gaza blockade was tragic but should be used as an opportunity to advance Middle East peace efforts.

A spokeswoman for activists aboard another boat trying to run the blockade said they would continue their voyage. Israel, facing an international outcry over its naval operation on Monday, has vowed to stop the MV Rachel Corrie, named after an activist killed by an Israeli bulldozer in Gaza in 2003.