Rail and Tube workers will stage strikes over the Bank Holiday weekend in separate disputes over pay, jobs and conditions.
RMT members at Heathrow Express will walk out for 24 hours from 3am on Friday, and again from the same time on Bank Holiday Monday. Unite union power control staff on London Underground will also strike for three days from Saturday in a dispute over pay and conditions.
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PC Keith Palmer, 48, stabbed by Khalid Masood while on duty in the grounds of the Palace of Westminster last Wednesday.
Those gathered on the bridge fell silent at 2.40pm - the time the first call was made to police exactly seven days ago.
Simon Lucy, 61, filmed hundreds of hours worth of footage on the London Underground network.