The worst flooding Thailand has seen in half a century reached outer Bangkok Friday as the prime minister declared the floods a natural disaster and asked the city’s 9 million residents to prepare to flee.

GALLERY: Click the image above to view photos of Thailand’s flooding.

In a risky move, Bangkok opened key gates to flood-control canals, hoping it would help drain high waters into the sea. If the move fails, however, floodwater could flow through the capitol’s streets.

Already affecting nearly 700,000 people and covering 12.5 percent of total crop land, the floods have devastated much of the country’s agriculture and industry. Flood damage is estimated at $6 billion and could rise much higher.

Watch al-Jazeera’s report on the crisis, where one witness describes the floods as coming in from “every direction”:

Many Bangkok residents have begun parking on the highway in preparation for the waters:

The last flooding of this magnitude happened in 1942, when Bangkok was hit with the “Great Flood.” Watch a clip from a documentary on the flooding, filled with images of the city under water: