LONDON, Feb. 20 (Xinhua) -- Hundreds of demonstrators gathered outside Britain's Houses of Parliament Monday as the debate over Brexit moved to the House of Lords.

With many carrying banners, they wanted to show their feelings about Britain's relationship spanning over 40 years with the European Union. Supporters of both Leave and Remain wanted to campaign for their respective sides as politicians packed the House of Lords.

British Prime Minister Theresa May made a rare appearance as she sat in the historic chamber to listen to the start of the debate over her parliamentary bill to trigger the process of Britain leaving the European Union.

Almost 200 peers in the upper chamber plan to speak in a two-day session, sitting until midnight Monday, with a second marathon session Tuesday.

May's Brexit bill has already cleared its crucial hurdle in the House of Commons where it sailed through with a massive majority.

In the House of Lords, there are more critics of Brexit, with the risk of peers putting forward changes to May's bill that could delay her aim to start triggering the Article 50 exit process by the end of next month.

May issued a message on Monday to peers ahead the debate. She said she did not want anybody to hold up what the people of Britain wanted following last June's national referendum which resulted in support for leaving the EU by a 52-48 majority.

Baroness Evans, leader of the House of Lords, opened the debate, calling on the House of Lords to respect the primacy of the House of Commons as well as the decision of the people of Britain in the referendum.

Labour's opposition leader in the House of Lords, Baroness Smith told peers the government would not be given a "blank cheque". She said she aims to make government ministers consider reasonable changes, but insisted this was not delaying the process but part of the Brexit process.

But Lord Newby, leader in the House of Lords of the of minority Liberal Democrats, described the government's approach to Brexit as little short of disastrous adding: "to sit on our hands in these circumstances is unthinkable and unconscionable".

Five days have been set aside for the so-called Article 50 bill to be debated and scrutinized by the House of Lords. It needs to clear this one final hurdle before it goes to Queen Elizabeth II to be given Royal assent.

Only then will May be certain she can tell Brussels that the exit process has started, launching two years of negotiations to work out a new relationship between Britain and the 27 member states of the EU.