With Labour searching for a new leader since Ed Miliband’s resignation, Angus Robertson said the SNP will be a strong voice for people in Scotland and across the UK.
The SNP are now the third biggest party in the House of Commons with 56 MPs taking their seats when Parliament resumes this week.
Mr Robertson said: “For the sake of people in Scotland and across the UK, it is vital that there is a strong and effective opposition to the Tory Government at Westminster - and that is a responsibility the SNP are ready and willing to discharge.
“We will work in partnership with the Scottish Government and Members of the Scottish Parliament, and are prepared to reach out across party lines at Westminster.
“Indeed, I believe that the SNP will form the real opposition to the Tories. Labour are leaderless at Westminster and mired in a leadership crisis in Scotland, but the issue is deeper than that.”
Mr Robertson claimed Labour are not offering an alternative to the Conservatives on austerity, welfare and Europe.
He added: “The SNP, by contrast, have a powerful manifesto mandate to oppose austerity, propose investment in job creation, speak out against cuts in benefits for disabled people, and insist that the UK cannot leave the European Union unless all four of its constituent parts agree.
“We will work closely with Ministers and MSPs in Holyrood on all of these and other issues - such as achieving the powers Scotland was promised in the referendum, and preventing the repeal of the Human Rights Act.
“Effective SNP opposition to the Tories will certainly be good for Scotland - but leading this progressive alternative will also be to the benefit of people throughout the UK.”