Centrist French presidential candidate Emmanuel Macron has said the remaining 27 European Union members must vigorously defend their single market in talks with Britain on its exit.
Mr Macron, presenting his policy platform for the election, also urged efforts to reinvigorate the eurozone and called for closer European co-operation.
He said the EU cannot survive "without a real European strategy" and called for a "new impulse for the single market".
Far-right candidate Marine Le Pen wants to pull France out of the EU and eurozone, and there has been growing anti-EU sentiment in many countries since Britain's vote to leave.
Polls suggest Mr Macron and Ms Le Pen may face off in the May 7 presidential run-off.
Amid growing French political scandals, Mr Macron also wants to shrink the size of parliament, introduce term limits and ban officials from hiring their family members.
He said he wants to "eradicate conflicts of interest".
Two of Mr Macron's chief rivals for the April-May two-round vote - conservative Francois Fillon and Ms Le Pen - are facing corruption investigations.
Mr Macron, a 39-year-old who has never held elected office, is presenting himself as a fresh face without political baggage.
His platform calls for cutting the size of both houses of parliament by a third, banning politicians from consulting activity and banning all officials from employing family members.
Mr Macron also called for an international "roadmap" to better fight Islamic extremism from the Middle East to Africa.
He called for increased military spending to 2% of GDP - as US and other Nato allies have long demanded.
Mr Macron said he would hire 10,000 more police and create 15,000 more places in prison and boost efforts to improve relations between police and minority youths in poor suburbs.
His critics on the right have called him too soft on security.