Military vehicles and tanks were seen on the streets of Bahrain's capital, Manama, early Thursday after security forces cracked down on protests, dpa reported.
Witnesses said they saw tanks near the city centre, which had been cleared of civilian traffic.
Police moved in on the protesters near the central Lulu, or Pearl, Square roundabout about 3 am (0000 GMT), using tear gas, opening fire and closing off the area, witnesses said.
As many as four people were killed and dozens injured in the crackdown and later clashes, witnesses and medical sources said.