An office building opposite the Houses of Parliament and above Westminster underground station was evacuated while security officials searched the area, a spokeswoman for London's Metropolitan Police said.
People trying to access offices in Portcullis House, which houses MPs' offices, committee rooms and restaurants, were also turned away.
Portcullis House sits on top of Westminster Underground station and is connected to the Palace of Westminster by an under-pass.
Police said the reason for evacuation was a "suspect package" but declined to give more information or say how the package ended in the top-security parliament building, the Telegraph reported.
Britain has raised its terror threat level to second-highest level in August, meaning a terror attack on UK soil was highly likely. The terror threat level was raised fearing the risks posed by ISIS fighters returning from Iraq and Syria, where they have occupied vast swathes of land and declared an Islamic Caliphate in June.
More than 500 UK citizens are reported to be fighting alongside the ISIS fighters in the Middle East.
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