German Chancellor Gerhard Schroeder has warned the US to back away from the possibility of military action against Iran over its nuclear programme.
Iran restarted nuclear research this week
His comments come a day after President Bush reiterated that force remained an option but only as a last resort.
Iran has resumed what it says is a civilian nuclear research programme but which the West fears could be used to develop nuclear arms.
Germany, France and the UK have led efforts to end the crisis peacefully.
Mr Schroeder's rejection of force came at the official launch of his party's election campaign.
The BBC's Ray Furlong - reporting from Hanover - says there was an echo of his last election campaign three years ago, when his steadfast opposition to the use of force against Iraq helped get him re-elected.
Mr Schroeder directly challenged Mr Bush's comment that "all options are on the table" over the Iran crisis.
"Let's take the military option off the table. We have seen it doesn't work," Mr Schroeder told Social Democrats at the rally in Hanover, to rapturous applause from the crowd.
Mr Schroeder said it remained important that Iran did not gain atomic weapons, and a strong negotiating position was important.
"The Europeans and the Americans are united in this goal," he said. "Up to now we were also united in the way to pursue this."
Schroeder wants military action "off the table"
Mr Schroeder reiterates his views in an interview to be published Sunday in the German weekly Bild am Sonntag, labelling military action "extremely dangerous".
"This is why I can with certainty exclude any participation by the German government under my direction," Mr Schroeder tells the paper.
Mr Schroeder was among Europe's sternest critics of the Iraq war, causing a bitter rift with the US which poisoned relations between the two countries.
His opposition, in tandem with that President Jacques Chirac's France, led to US defence secretary Donald Rumsfeld's stinging attack on "old Europe".
NUCLEAR FUEL CYCLE
Mined uranium ore is purified and reconstituted into solid form known as yellowcake
The yellowcake is then reacted with acid is then converted into a gas by heating it to about 64C (147F)
Gas is fed through centrifuges, where its isotopes separate and process is repeated until uranium is enriched
Low-level enriched uranium is used for nuclear fuel
Highly enriched uranium can be used in nuclear weapons
The UN's atomic watchdog, the International Atomic Energy Agency, backed a resolution this week expressing "serious concern" at the resumption of the nuclear programme, and demanding it be halted again at once.
Mr Bush's comments about the military option came in an interview on Israeli TV.
The BBC's Jonathan Beale in Washington says the president wants to send a clear warning to Tehran, although in reality the US already has its hands full in neighbouring Iraq.
Mr Schroeder is lagging well behind his conservative rivals in the German election campaign, but has been narrowing the gap in recent days.
In the 2002 poll, he came from behind to snatch victory after anti-Iraq war feeling - and an outbreak of serious flooding in Germany - helped him attract last-minute support.