The M25 in Essex is closed in both directions between J23 (A1) - J27 (M11) clockwise, and J27 (M11) - J25 (Cheshunt) anti clockwise. The carriageway may not fully reopen until 3pm.
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Worshippers tackled a knifeman in a mosque after he stabbed a man in his 70s who was leading the call to prayer.
It is believed to be the second time the Metropolitan Police have used the surveillance which has been criticised by privacy campaigners.
Police were called to the prison on Tuesday and two inmates, both 28-year-old men, were arrested on suspicion of attempted murder.