Two men were released earlier in the week.
Police raided 18 homes and offices in Birmingham as part of "operation gamble".
'Police state for Muslims'
UK media reports allege the kidnappers intended to behead their intended victim, a British Muslim soldier, and then post a video of the execution on the internet.
Abu Bakr, one of the two men released earlier in the week, said Britain was now a "police state" for Muslims.
In an interview with the BBC he said that British terror laws "are designed specifically for Muslims".
"We are feeling the brunt of it all," he said.
After the arrests, the British government came under criticism from civil rights groups over apparent leaks to the media regarding the raids and subsequent investigation.
Britain has been on its second-highest threat level of "severe", indicating that security services believe a terrorist attack is highly likely.
British police are allowed to detain terrorism suspects for a total of 28 days without charging them, subject to regular court approval.