European Central Bank president Jean-Claude Trichet was hammered by analysts and media on Friday after an ECB press conference at which he tried to calm market turmoil fuelled by the Greek debt crisis.

Fears the bank is under increasing political pressure lingered following what Goldman Sachs chief European economist Erik Nielsen called "one of the most depressing" briefings in the bank's history.

UniCredit chief economist Marco Annunziata said that Trichet was, somewhat excessively, "put on trial for his previous statements on collateral rules and on the possible IMF involvement in a Greek rescue package."

But the economist also warned that the central bank's communication policy had suffered by "becoming embroiled in political discussions that sometimes defy logic," and said the bank was at threat of "further damaging its credibility."

Annunziata urged the ECB to "take a big step back."

The assessments are important because the eurozone is based heavily on the independence of the ECB from any political pressure over monetary policy, and on the strong credibility of the central bank in ensuring stability in the zone.

Many observers said they felt that Trichet had made two surprising U-turns, on the bank's opposition to possible International Monetary Fund support for Greece, and on the complex issue of collateral criteria set by the ECB for loans by the central bank to commercial banks, and notably Greek banks.

The rules on loan collateral are crucial for Greece because the criteria will determine whether Greek government bonds are accepted by the ECB as collateral next year.

Greek banks are particularly dependent on ECB funding, and if they could not offer Greek government bonds as collateral for loans, they would suffer severe penalties in terms of commercial borrowing costs and also of credibility.

Trichet had said initially that criteria would not be set to suit any single country, suggesting a hard line with respect to Greek bonds, but it was clear on Thursday that Greek sovereign debt would continue be accepted, albeit at a discount, meaning less favourable terms.

Barclays Capital economist Thorsten Polleit told AFP before Trichet's press conference that a clear explanation for the U-turns was needed since investors felt the bank was under pressure to ensure that Greek banks continued to obtain funding.

"There is of course political pressure now, and Greece may be a precedent" if problems funding debt crop up in other eurozone countries such as Portugal or Spain, Polleit told AFP on Friday.

Polleit said: "the latest decisions are indicative that this political pressure is going to have an effect on (ECB) policy decisions."

ING senior economist Carsten Brzeski also said that "Trichet had a few weak moments and did not really succeed in convincingly explaining the ECB's U-turn" on IMF involvement in a Greek rescue, which ECB directors had firmly opposed.

German business daily Handelsblatt commented that "in the Greek crisis the ECB is demonstrating that it is following political aims rather than the stability culture of its founders."

Germany backed the eurozone's creation on condition it respected strict inflation standards of its own central bank, but a Bundesbank note quoted Thursday by the Financial Times dubbed the IMF an "Inflation Maximizing Fund."

The IMF's chief economist has called for allowing a higher inflation target than the one accepted by the Bundesbank and the ECB.

Handelsblatt editor-in-chief Gabor Steingart said Friday that "Trichet wants to save Greece, he wants to revive the economy, but above all he wants to avoid a row with politicians.

"Yesterday he kissed inflation and for the spirit of the Bundesbank, this was the kiss of death."

In Athens, the Ta Nea newspaper commented in a headline that Trichet's remarks on Thursday had thrown Greece a "lifebelt."