Traffic has been brought to a crawl on many Bahrain highways after calls by pro-reform groups to flood the roads with cars in a show of strength before parliamentary elections later this week.
Shiite-led protesters are seeking to send a message of defiance to Sunni authorities after warnings of harsh responses to any attempts at disrupting Saturday's voting in the Gulf kingdom.
The traffic jams hit during the morning commute Wednesday. Security forces were on high alert, but there were no reports of violence.
Bahrain's majority Shiites began protests in February for greater rights from the ruling Sunni dynasty. Shiite groups are urging a boycott of the upcoming election for 18 seats abandoned by Shiite lawmakers to protest the crackdown.