"We have around 250 confirmed arrested and 44 who are missing, though that number fluctuates when people reappear after hiding from police," Ibrahim Mattar told Reuters on Monday.
Mattar, who is from the largest opposition group al-Wefaq, added that many Bahrainis are being arrested at checkpoints or in house raids.
Most of those who were detained or went missing were not activists, he noted.
On Sunday, protesters once again poured into the streets of the capital city, Manama, despite the state of emergency imposed by King Hamad bin Al Khalifa on March 15.
Bahraini forces along with troops from Saudi Arabia and the United Arab Emirates have stepped up their attacks against protesters.
The protests against the government began in mid-February. At least 24 people have been martyred and about 1,000 others have been injured so far.
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