WASHINGTON (AFP) - Pope Benedict XVI tweeted his best wishes Friday to hundreds of thousands of pro-life supporters converging on Washington 40 years after the landmark US Supreme Court ruling that legalised abortion.

Organisers of the annual March for Life said they hope the crowd along the National Mall will surpass last year's record estimated turnout of 400,000, and police have sealed off streets along the march route.

"I join all those marching for life from afar, and pray that political leaders will protect the unborn and promote a culture of life," Pope Benedict tweeted on his @Pontifex account.

In Roe versus Wade, the Supreme Court effectively legalised abortion in the US by stating that the termination of a pregnancy was a strictly private matter between a woman and her doctor.

Public opinion polls find the nation as divided as ever on the issue, however, with the Roman Catholic and Evangelicalist churches at the forefront of the pro-life cause.

Pro-choice activists held a smaller event at the steps of the Supreme Court earlier this week.