Peers have been urged to “respect” voters’ decision to leave the EU as they began debating the Brexit bill.
With Prime Minister Theresa May taking the unusual step of sitting in the Lords to watch the opening speeches, Leader of the House Baroness Evans said peers must not “frustrate” Brexit.
But Labour said “reasonable changes” could take place to the bill.
MPs have already backed the proposed law, authorising Mrs May to inform the EU of the UK’s intention to leave.
The government does not have a majority in the House of Lords where a record 190 peers are due to speak over two days.
The sitting was extended to midnight on Monday to allow more peers to speak and the debate will continue on Tuesday.
Opposition and crossbench peers are seeking guarantees about the rights of EU citizens in Britain and the role of parliament in scrutinising the process.
Mrs May has said she wants to invoke Article 50 of the 2009 Lisbon Treaty – the formal two-year mechanism by which a state must leave the EU – by the end of March, and the government has warned the House of Lords not to frustrate the process.