British far-right leader Nick Griffin was among a group of European politicians visiting Syria on Tuesday at the invitation of President Bashar al-Assad's regime.
British National Party (BNP) spokesman Simon Darby said Griffin and other European delegates had been invited by the Syrian foreign ministry.
Griffin, a member of the European Parliament, said he wanted to highlight the risk that any future British move to arm the rebels fighting Assad could plunge Syria into an "Iraq-style hell of sectarian hate".
The BNP leader also linked Islamist militancy espoused by some of the rebels with the hacking to death of British soldier Lee Rigby in London last month.
Griffin, 54, entered Syria from neighbouring Lebanon and posted a series of comments about the trip on Twitter.
"Now in Damascus, which is busy, a modern, bustling city. Security rather like Belfast in Troubles," he wrote, referring to the years of violent conflict in Northern Ireland.
"Fact finding mission. (British Prime Minister David) Cameron and (Foreign Secretary William) Hague plan to send UK money and weapons to rebels dominated by Islamist jihadis like killers of Lee Rigby.
"Occasional explosions in distance but life in capital normal. Traffic busy, shops full of goods. Families out in sun. Why turn stable secular state into Iraq-style hell of sectarian hate? More madness from the people who dragged us to costly war in Iraq and Afghan(istan).
"Just visited site of rebel suicide bomb attack this a.m. Smelt like abattoir. Vile. Hague wants your taxes to arm these terrorists!" he added.
Britain successfully pushed for the European Union to lift its arms embargo on Syria at the end of May, but has not yet taken a decision on arming the rebels.
Darby said Hague was pushing Britain "into conflict and supporting people that we know very little about", adding that getting involved in other countries was "none of our business".
The BNP came fifth at the 2010 general election with 1.9 percent of the vote.