British police have evacuated 20,000 people from the central city of Birmingham, after intelligence suggested a security threat.

Police on Saturday said they do not think the threat to England's second-biggest city is linked to Thursday's bombings in London. No specific information about the threat to Birmingham was released.

Located 175 kilometers northwest of the capital, Birmingham has a lively entertainment district, packed with restaurants and nightclubs.

In London, British authorities say the four bombs that devastated three subway trains and one bus Thursday were high explosives, not homemade devices. They note the explosions were almost simultaneous, and were likely set off by timing devices.

Recovery crews are working to extricate bodies still trapped in a subway tunnel.

British officials say the London attacks bear the hallmarks of the al-Qaida terrorist network.

Some information for this report provided by AFP and Reuters.