Over 4,000 cities in 131 countries around the world - including New Delhi and Mumbai - will turn off their lights this evening for Earth Hour, which is observed annually on the last Saturday of March under the aegis of the Worldwide Fund for Nature.
"Earth Hour is your chance to vote for earth by shutting down your lights between 8.30 pm and 9.30 pm your local time," the WWF said. The hour officially begins at 2030 GMT.
In India, not only the major metropolises but also so-called tier-II cities like Ahmedabad and Pune - to name just two - have officially declared that they will participate. But, basically, in India, this only means that external lights in government buildings and installations will be turned off.
In other cities internationally, participation is likely to be much more widespread. Moscow, Geneva, Budapest, Berlin, Rome, Barcelona, Helsinki, Oslo, Madrid, Paris, London, Amsterdam and Vienna are some European cities that will turn off all non-essential lights.
In Asia, cities like Bangkok, Singapore, Ho Chi Minh City, Hong Kong, Seoul, Auckland and Shanghai will support the WWF initiative.
In North America, Toronto, Vancouver, New York and Chicago are among those that will go dark. In the South, cities including Rio de Janeiro, Buenos Aires, Santiago, Sao Paulo, and Valparaiso will take part.
Even the Davis research station in Antarctica will turn off its lights.