Barack Obama has taken to Twitter to thank his supporters for turning out to vote and returning him to office for another four years.

At the time of writing, all the major American news networks are calling the presidential election for Obama, with a number of key swing states projected to fall to the Democrats.

At around 11.15pm ET, Obama’s campaign team tweeted, “This happened because of you. Thank you.”

Another came a minute later: “We’re all in this together. That’s how we campaigned, and that’s who we are. Thank you. -bo” The inclusion of -bo indicates a personal tweet from Obama himself.

The campaign team then tweeted a photo (below) of Obama hugging wife Michelle, accompanied by the words, “Four more years.” Twitter has just reported that this has already become @BarackObama’s most retweeted tweet ever – at the time of writing this stands at 314,000 – not bad for a tweet posted 40 minutes ago. Mind you, the account does have 22.1 million followers.

However, Mitt Romney’s Twitter account has so far stayed silent this evening, with the last tweet posted around 3pm ET. “With your help, we will turn our country around and get America back on the path to prosperity. Please vote today,” it said.

Earlier in the evening, Twitter announced that Election Day had already become the most tweeted about event in US political history.

[Update: Obama's tweet has now overtaken a Justin Bieber tweet to become the most retweeted ever.]