Many former British servicemen, some decorated with medals and former prison camp badges, jeered as the Emperor and his wife - accompanied by the Prince of Wales - were driven through the castle gates where they met local dignitaries and were entertained by a programme of traditional Welsh music from the Mass Choir of the Welsh Association of Male Voice Choirs.
The protest by Welsh war veterans followed a similar demonstration in London on Tuesday when hundreds of PoWs turned their backs on the Emperor and his wife as they rode by with the Queen in a royal carriage.
One of those who attended yesterday's protest, Wyndham Jeremiah, 78, from Newbridge, said he was pleased with the way it had been carried out. "It is the only form of protest we have got. I was a bit surprised about the booing but turning your back on the Japanese is an insult and that's what we wanted to do. We ex-servicemen usually protest in a quiet way."
Oliver Davis, 82, also from Newbridge, said he felt he had done his bit in protesting at the Emperor's visit. He said: "I wasn't in the Far East but I am here for those who were. I was in the Royal Pioneer Corps in Europe but I felt it was my duty to be present today.
"I don't think we will achieve anything but it is still good to make a stand."
One former PoW bumped into the widow of a fellow serviceman who spent three and a half years with him in camps in the Far East.
Glyn Thomas, 76, from Neath, recognised the number on a prison camp ID badge pinned to the jacket of his fellow inmate's widow.
Monica Yabsley was at the protest on behalf of her husband Ray, an ex- RAF man who died 11 years ago.
She said: "It was lovely to meet someone who went through the same thing as my husband."
Japanese teacher Sachi Ebisu, 21, from Osaka, said she was "saddened" by the protest.
Currently teaching Japanese in Cardiff, she said: "I hope this doesn't strain relations between our two countries.
"I do not think the Emperor has the responsibility for what happened during the war because he was just a little boy. Many soldiers used the name of the Emperor, but after the war they were killed for the crime of abusing his name.
"I feel very embarrassed about this insult to him."
A handful of war veterans held up a Union flag and turned their backs as the Emperor and Empress left the castle yesterday afternoon.
Several hundred spectators raised a muted cheer as the motorcade drove away. Banners were held up bearing the messages "apologise", written in Japanese, and "compensate our ex-PoWs".
Liberal Democrat MP Mark Oaten, co-chairman of the all-party parliamentary group on Far East prisoners of war, claimed yesterday that the Emperor had moved closer towards an apology.
Mr Oaten said: "The Emperor's sympathy with former PoWs is becoming increasingly clear as his visit progresses.
"Inch by inch, the Emperor and his entourage have moved towards an apology, and I congratulate them on this sympathetic approach.
"Now that the Emperor has seen at first hand the strength of feeling that exists in this country, he surely must make his government realise that unless there is a clear apology from them, there will always be a thorn in the side of Anglo-Japanese relations."Reuse content