Britain raised the threat level to "critical" — it's the highest level, meaning more attacks may be imminent.
BBC: "[M]ilitary personnel are now being deployed to protect key sites. The Palace of Westminster has been closed to the public following police advice, and will not re-open until further notice ... [T]he Changing the Guard ceremony at Buckingham Palace has been cancelled on Wednesday to redeploy police officers."
"The man who carried out a suicide attack in Manchester was "likely" to have not acted alone ... Police arrested three men in south Manchester" today.