The First Minister was forced to defend his blueprint for a seamless transition to EU statehood during yesterday's question time at the Scottish Parliament.
He accused his critics of "scaremongering" over the future of the country.
Labour, Conservative and Liberal Democrat leaders all weighed in to attempt to undermine one of the SNP's central promises, that Scotland would be able to negotiate EU entry from inside the UK, all within 18 months.
The political row erupted after Spanish leader Mariano Rajoy reportedly said: "If part of a member state becomes independent, it would be left out of the EU and it would be good for citizens (in the EU) and Scots to know that."
Mr Salmond tried to defend his approach by highlighting a letter, lifted from the internet, which states it is "legally possible" to negotiate within the EU if all states agree.
The letter, sent to an unknown person, was signed by European Commission (EC) official Mario Tenreiro.
Labour leader Johann Lamont said: "This is an amateurish and shameful attempt by Alex Salmond to mislead the people of Scotland.
"It seems that Alex Salmond is basing Scotland's future on his ability to Google."
Downing Street said Mr Rajoy's comments echoed legal advice the British Government had published earlier this year.
The Prime Minister's official spokesman said: "I think prime minister Rajoy has made a very important point.
"I believe it is very much consistent with the legal advice that the British Government has published."
Mr Salmond said his position is backed by the EC letter, and by Sir David Edward, a former judge of the European Court of Justice. He also cited Professor James Crawford, who was appointed and paid by the UK Government to provide legal advice on the issue.
He said: "Scotland is a European nation. Anybody with an ounce of sense knows that. Can Better Together not just get off the scaremongering and embrace the idea that this nation is a European nation?"
SNP MEP Ian Hudghton suggested that the Spanish government has reasons to make transition appear difficult because of Catalan attempts to win independence.
The Scottish Government published a paper devoted to EU membership on Wednesday, one day after the White Paper was unveiled.