Burmese opposition leader Aung San Suu Kyi appeared "composed" and "crackling with energy" at a trial hearing, a diplomat who attended the session says.
UK ambassador Mark Canning welcomed the decision to let diplomats and journalists observe the trial, but said he had no confidence in the outcome.
The trial, which has drawn widespread international criticism, began on Monday with no outside observers.
Ms Suu Kyi is charged with breaking the terms of her house arrest.
The pro-democracy leader, who is 63 and has been in frail health, has been in detention without being charged for more than 13 of the past 19 years.
She is currently being detained in an annex of Rangoon's notorious Insein prison.
Ms Suu Kyi's latest period of house arrest was scheduled to expire on 27 May, and many observers see this case against her as a pretext to ensure she is still in detention during next year's elections.
The decision to let a group of journalists and diplomats into Ms Suu Kyi's trial hearing on Wednesday afternoon was seen as a rare concession by the Burmese regime to international opinion.
During the hearing, which lasted for about an hour, a police officer testified for the prosecution, Mr Canning told the BBC.
"She was composed, upright, crackling with energy," he said. "Very much in charge of her defence team."
I think this is a story where the conclusion is already scripted
Afterwards, she spoke briefly to diplomats to welcome their presence and say she hoped she would meet them in better times, he said.
She then held a separate meeting with three foreign officials: the ambassador of Singapore - the longest serving envoy in Burma - and representatives from Russia, which currently chairs the UN Security Council, and Thailand.
Ms Suu Kyi told the officials that she and her two housekeepers were being treated well and that "it was not too late for something good to come out of this unfortunate incident", Singapore's foreign ministry reported.
Mr Canning said it was not clear whether access to the trial would continue.
"I think this is a story where the conclusion is already scripted," he said.
"I don't have any confidence in the outcome. While the access we had today was very welcome, it doesn't change the fundamental problem."
Earlier, officials announced that 10 journalists from local and foreign news organisations would be able cover the hearing, but that photos and video footage would still be banned.
AUNG SAN SUU KYI
1988: Military junta comes to power after crushing pro-democracy uprising
1989: Martial law declared; opposition NLD leader Aung San Suu Kyi put under house arrest
1990: NLD wins elections; result rejected by the ruling junta
1995: Suu Kyi released from house arrest, but movements restricted
Sept 2000: Put under house arrest again when she tried to defy travel restrictions
May 2002: Released unconditionally
May 2003: Detained after clash between NLD and government forces
Sept 2003: Allowed home after operation, but under effective house arrest. In the years since, the orders for her detention periodically renewed
Embassies were informed that they could send one official - a change from the situation on Monday, when several European diplomats tried to enter the trial but were denied access.
The BBC's South East Asia correspondent Jonathan Head says that holding the hearings in secret, inside the country's top-security prison, has provoked particular condemnation.
On Tuesday South East Asian leaders expressed "grave concern" about the trial.
The statement from the Asean group warned that the "honour and credibility" of Burma's government were at stake.
Ms Suu Kyi, along with two female assistants, is accused of violating the terms of her house arrest after a US man, John Yettaw, swam to her lakeside home earlier this month.
Mr Yettaw is also facing charges.
The government plans to call 22 witnesses to support its case.
Some have already testified that Mr Yettaw swam across the lake that backs onto Ms Suu Kyi's home, and was arrested as he left, after being allowed to stay two nights.
Ms Suu Kyi's lawyer has argued that he was uninvited, and was only allowed to stay after pleading exhaustion from his midnight swim.
Ms Suu Kyi's National League for Democracy (NLD) party won democratic elections in 1990, but the military never allowed the party to govern.