Section 12. The taking shall become absolute and the right of the body politic to the real estate or interest therein described in the petition and the right of the respondents to compensation or damages shall vest upon the entry of either final or interlocutory judgment of condemnation, and it shall have immediate right of entry and possession, subject to the rights of the owners under section two with respect to structures and trees. The compensation or damages shall be payable when vested, or as soon thereafter as they are finally determined, and if not then paid may be recovered in an action of contract. In case two or more persons have appeared in the proceedings as parties respondent with interests adverse to each other with respect to a particular parcel of land, the body politic shall not be bound to pay such compensation until one or more of such persons have established his or their rights thereto in proper judicial proceedings to which the others of such persons are parties; but the proceedings to determine the amount of compensation shall not be stayed pending the establishment of such rights. Compensation and damages shall be assessed as of the date of the adoption of the order, and shall bear interest at the rate of six per cent per annum from the date when the right to damages becomes vested until payable, but shall not bear interest after they are payable unless the body politic fails upon demand to pay the same to the person entitled thereto. The body politic shall not pay compensation or damages to any person in excess of its estimate, except in accordance with an award of the commissioners, nor in excess of the award of the commissioners, except as amended in accordance with a verdict of the jury.