* 150 member body met for first time in Geneva
* U.N. envoy says talks sometimes "very painful"
* No agreement yet on draft constitution, detainees (Adds details throughout)
By Emma Farge and Babak Dehghanpisheh
GENEVA, Nov 8 (Reuters) - The opening round of the first Syrian peace talks in more than a year went "better than most people would have expected", a U.N. envoy said, although delegates described a chilly atmosphere with those from opposing sides not yet shaking hands.
Representatives of President Bashar al-Assad's government and the opposition met in Geneva to discuss a future constitution, part of plans for a political settlement to end 8 1/2 years of war.
Expectations for the talks have been low, with Damascus and its Iranian and Russian allies having made gains on the battlefield that left them few reasons to grant concessions.
Acknowledging that discussions were sometimes difficult, U.N. envoy Geir Pedersen said delegates from the government, opposition and civil society had behaved with professionalism. Talks would resume in Geneva on Nov. 25.
"These are sometimes very painful discussions and it takes courage to listen to the other side defending its views on these issues," he told journalists in Geneva.
"I believe it has gone much better than most people would have expected," he said.
The government delegation had been seeking to hold the next round of talks in Syria's capital, which the opposition had strongly resisted.
The talks are focused on drawing up a constitution with a view to eventually holding elections in Syria, a less sweeping agenda than at U.N.-sponsored talks earlier in the war, which collapsed over rebel demands that Assad leave power.
Rebel fighters who once aimed to overthrow Assad now hold only a small pocket of northwestern Syria and have been driven out of all its major population centres.
Still, it has taken nearly two years to form the committee amid wrangling discussions over its composition. In Geneva last week, the 150 delegates agreed the composition of a smaller 45-member drafting body tasked with writing a draft constitution that would be eventually presented to Syrian voters.
After ten days of talks, there was no immediate agreement on the release of thousands of detainees, an issue that Pedersen has underscored as key to building confidence.
Nor was there consensus on whether delegates from the so-called small group in charge of drafting the constitution would adapt a 2012 constitution or start afresh with a new one.
Addressing journalists, Ahmad Kuzbari, co-chair of the panel for the government and a member of Syria's parliament, appeared to rule out any outcome that would overhaul the status quo.
"We did not come to build a new state. The Syrian Arab Republic is a state: it has a constitution, a parliament, a government, an army and institutions."
Talks between the parties were often heated, delegates said, particularly on the issue of "terrorism", a term used by the government side to refer to insurgents, with the government body seeking to incorporate this within the constitutional reform project. The opposition side resisted this.
"Discussions were not very easy," the opposition co-chair Hadi al-Bahra said, noting that he had yet to shake hands with his counterpart. "We all had to be reasonable and go beyond the differences and to focus on the parts that unify Syrians." (Reporting by Emma Farge and Babak Dehghanpisheh Additional reporting by Kinda Makieh Editing by Peter Graff)