Plaid Cymru President Ieuan Wyn Jones opened his speech to the party conference by calling the terror attacks in America crimes against humanity.

He said all were "agreed that those responsible for these evil deeds must be found and brought to justice".

But he called on the West to include the Muslim world in its planning to strike back and not to respond with an indiscriminate reprisal which would lead to the loss of thousands of innocent lives.

He said: "No words can condemn such appalling crimes against humanity."

"Increased co-operation at a global level is the only way forward for us if we are to prevent similar atrocities in the future.

Plaid Cymru's 2001 conference in Cardiff

"And above all, the West must not alienate those millions of Muslims throughout the world who share our horror at these attacks."

But Mr Jones also used his leader's address to return to the more domestic problems which have affected his party's popularity - rows over the future of the Welsh language and the attitude of some in his party to incomers to Wales.

He said there can be no denying that communities throughout Wales are facing great threats but that there there were massive opportunities as well.

"The worst thing we could do would be to portray migration to Wales simply in terms of a threat. That would create a destructive climate in every way," he said.

He said: "This is not to ignore the fact that some incomers do have a somewhat colonial attitude - and who are disrespectful of the people they came to live in their midst.

"And there have even been examples of employers with the effrontery to deny their workers the right to speak Welsh at the workplace.

"This is totally unacceptable, and must be dealt with firmly and determinedly."

But, while admitting that the party's vote had decreased in three of its strongest seats in June's General Election, he issued what will be seen as a rallying call to the grassroots of party to stick with Plaid over this issue.

"A perception has been created in some quarters that we are no longer committed to the Welsh speaking communities of the north and the west.

Distinct culture

"Let me take this opportunity to quash that perception once and for all.

"We remain totally committed to them and our commitent to communities in the rest of Wales is none the less for that."

He also said that the challenge to those who have moved to Wales mainly from England is to understand the country into which they have moved and to recognise the distinct culture and values of Wales.

On Thursday, the party passed a motion on the de-criminalisation of cannabis for medicinal and recreational use.

It was a departure from Plaid's former policy of calling for a Royal Commission on the issue - the party is now the only one of the four main parties to support the liberalisation of cannabis laws.

The conference at St David's Hall in Cardiff held a minute's silence as a mark of respect for the thousands killed in the suicide hijacking and crashing of four American passenger jet liners.