* Parts of Stockholm sealed off for hours
* Police says negotiating with man claiming to carry explosives
* Threats made against Social Democrats, Moderates (Adds police comment, detail)
STOCKHOLM, June 19 (Reuters) - Large parts of central Stockholm, including the headquarters of Sweden's Social Democrat Party and the ruling Moderates, were sealed off on Thursday as police investigated a bomb threat.
Stockholm police said a man had barricaded himself in a building near the Moderates' headquarters, which had since been evacuated.
"A man walked into a human rights organisation at Stora Nygatan 26 at 1424 (1224 GMT) today, saying he was carrying explosive substances," spokesman Sven-Erik Olsson said.
"He expressed threats against the Moderates and the Social Democrats and we have therefore sealed off the areas around the Moderates' headquarters in the Old Town and the Social Democrats' headquarters at Sveavagen."
Swedish news agency TT reported, not revealing its sources, that the man was said to be wearing a bomb belt.
The police said they were working on bringing about a negotiated solution, although Swedish television quoted the head of the police negotiating team as saying they had not yet spoken to the man.
The Social Democrats and the Moderates declined to comment.
(Reporting by Johan Sennero and Anna Ringstrom; Editing by Alistair Scrutton and Mark Trevelyan)